diff --git a/OpenSim/Region/CoreModules/Avatar/Attachments/AttachmentsModule.cs b/OpenSim/Region/CoreModules/Avatar/Attachments/AttachmentsModule.cs
index 9eb0e383c0..eccf7a6977 100644
--- a/OpenSim/Region/CoreModules/Avatar/Attachments/AttachmentsModule.cs
+++ b/OpenSim/Region/CoreModules/Avatar/Attachments/AttachmentsModule.cs
@@ -251,6 +251,15 @@ namespace OpenSim.Region.CoreModules.Avatar.Attachments
 //                m_log.DebugFormat(
 //                    "[ATTACHMENTS MODULE]: Attaching object {0} {1} to {2} point {3} from ground (silent = {4})",
 //                    group.Name, group.LocalId, sp.Name, attachmentPt, silent);
+
+                if (group.GetSittingAvatarsCount() != 0)
+                {
+//                    m_log.WarnFormat(
+//                        "[ATTACHMENTS MODULE]: Ignoring request to attach {0} {1} to {2} on {3} since {4} avatars are still sitting on it",
+//                        group.Name, group.LocalId, sp.Name, attachmentPt, group.GetSittingAvatarsCount());
+    
+                    return false;
+                }
     
                 if (sp.GetAttachments(attachmentPt).Contains(group))
                 {
diff --git a/OpenSim/Region/CoreModules/Avatar/Attachments/Tests/AttachmentsModuleTests.cs b/OpenSim/Region/CoreModules/Avatar/Attachments/Tests/AttachmentsModuleTests.cs
index 5dcbd28f59..3e06900648 100644
--- a/OpenSim/Region/CoreModules/Avatar/Attachments/Tests/AttachmentsModuleTests.cs
+++ b/OpenSim/Region/CoreModules/Avatar/Attachments/Tests/AttachmentsModuleTests.cs
@@ -118,7 +118,7 @@ namespace OpenSim.Region.CoreModules.Avatar.Attachments.Tests
 
             Scene scene = CreateDefaultTestScene();
             UserAccount ua1 = UserAccountHelpers.CreateUserWithInventory(scene, 0x1);
-            ScenePresence sp = SceneHelpers.AddScenePresence(scene, ua1.PrincipalID);
+            ScenePresence sp = SceneHelpers.AddScenePresence(scene, ua1);
 
             string attName = "att";
 
@@ -154,6 +154,36 @@ namespace OpenSim.Region.CoreModules.Avatar.Attachments.Tests
 //            TestHelpers.DisableLogging();
         }
 
+        /// <summary>
+        /// Test that we do not attempt to attach an in-world object that someone else is sitting on.
+        /// </summary>
+        [Test]
+        public void TestAddSatOnAttachmentFromGround()
+        {
+            TestHelpers.InMethod();
+//            TestHelpers.EnableLogging();
+
+            Scene scene = CreateDefaultTestScene();
+            UserAccount ua1 = UserAccountHelpers.CreateUserWithInventory(scene, 0x1);
+            ScenePresence sp = SceneHelpers.AddScenePresence(scene, ua1);
+
+            string attName = "att";
+
+            SceneObjectGroup so = SceneHelpers.AddSceneObject(scene, attName, sp.UUID);
+
+            UserAccount ua2 = UserAccountHelpers.CreateUserWithInventory(scene, 0x2);
+            ScenePresence sp2 = SceneHelpers.AddScenePresence(scene, ua2);
+
+            // Put avatar within 10m of the prim so that sit doesn't fail.
+            sp2.AbsolutePosition = new Vector3(0, 0, 0);
+            sp2.HandleAgentRequestSit(sp2.ControllingClient, sp2.UUID, so.UUID, Vector3.Zero);
+
+            scene.AttachmentsModule.AttachObject(sp, so, (uint)AttachmentPoint.Chest, false);
+
+            Assert.That(sp.HasAttachments(), Is.False);
+            Assert.That(scene.GetSceneObjectGroups().Count, Is.EqualTo(1));
+        }
+
         [Test]
         public void TestAddAttachmentFromInventory()
         {
diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
index 4e0e183b5f..fc0476157f 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs
@@ -3401,6 +3401,20 @@ namespace OpenSim.Region.Framework.Scenes
             return count;
         }
 
+        /// <summary>
+        /// Gets the number of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public int GetSittingAvatarsCount()
+        {
+             int count = 0;
+
+             Array.ForEach<SceneObjectPart>(m_parts.GetArray(), p => count += p.GetSittingAvatarsCount());
+
+             return count;
+        }
+
         public override string ToString()
         {
             return String.Format("{0} {1} ({2})", Name, UUID, AbsolutePosition);
diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs b/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
index 3d8135826b..6518b84e49 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
@@ -374,7 +374,6 @@ namespace OpenSim.Region.Framework.Scenes
         private uint _category;
         private Int32 _creationDate;
         private uint _parentID = 0;
-        private UUID m_sitTargetAvatar = UUID.Zero;
         private uint _baseMask = (uint)PermissionMask.All;
         private uint _ownerMask = (uint)PermissionMask.All;
         private uint _groupMask = (uint)PermissionMask.None;
@@ -1233,13 +1232,20 @@ namespace OpenSim.Region.Framework.Scenes
         }
 
         /// <summary>
-        /// ID of the avatar that is sat on us.  If there is no such avatar then is UUID.Zero
+        /// ID of the avatar that is sat on us if we have a sit target.  If there is no such avatar then is UUID.Zero
         /// </summary>
-        public UUID SitTargetAvatar
-        {
-            get { return m_sitTargetAvatar; }
-            set { m_sitTargetAvatar = value; }
-        }
+        public UUID SitTargetAvatar { get; set; }
+
+        /// <summary>
+        /// IDs of all avatars start on this object part.
+        /// </summary>
+        /// <remarks>
+        /// We need to track this so that we can stop sat upon prims from being attached.
+        /// </remarks>
+        /// <value>
+        /// null if there are no sitting avatars.  This is to save us create a hashset for every prim in a scene.
+        /// </value>
+        private HashSet<UUID> m_sittingAvatars;
 
         public virtual UUID RegionID
         {
@@ -4493,5 +4499,83 @@ namespace OpenSim.Region.Framework.Scenes
             Color color = Color;
             return new Color4(color.R, color.G, color.B, (byte)(0xFF - color.A));
         }
+
+        /// <summary>
+        /// Record an avatar sitting on this part.
+        /// </summary>
+        /// <remarks>This is called for all the sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns>
+        /// true if the avatar was not already recorded, false otherwise.
+        /// </returns>
+        /// <param name='avatarId'></param>
+        protected internal bool AddSittingAvatar(UUID avatarId)
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            if (sittingAvatars == null)
+                sittingAvatars = new HashSet<UUID>();
+
+            lock (sittingAvatars)
+            {
+                m_sittingAvatars = sittingAvatars;
+                return m_sittingAvatars.Add(avatarId);
+            }
+        }
+
+        /// <summary>
+        /// Remove an avatar recorded as sitting on this part.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns>
+        /// true if the avatar was present and removed, false if it was not present.
+        /// </returns>
+        /// <param name='avatarId'></param>
+        protected internal bool RemoveSittingAvatar(UUID avatarId)
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            // This can occur under a race condition where another thread
+            if (sittingAvatars == null)
+                return false;
+
+            lock (sittingAvatars)
+            {
+                if (sittingAvatars.Remove(avatarId))
+                {
+                    if (sittingAvatars.Count == 0)
+                        m_sittingAvatars = null;
+
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Get a copy of the list of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public HashSet<UUID> GetSittingAvatars()
+        {
+            return new HashSet<UUID>(m_sittingAvatars);
+        }
+
+        /// <summary>
+        /// Gets the number of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public int GetSittingAvatarsCount()
+        {
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            if (sittingAvatars == null)
+                return 0;
+
+            lock (sittingAvatars)
+                return sittingAvatars.Count;
+        }
     }
 }
diff --git a/OpenSim/Region/Framework/Scenes/ScenePresence.cs b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
index c7a670f2c2..c6a2a032dc 100644
--- a/OpenSim/Region/Framework/Scenes/ScenePresence.cs
+++ b/OpenSim/Region/Framework/Scenes/ScenePresence.cs
@@ -577,6 +577,12 @@ namespace OpenSim.Region.Framework.Scenes
         /// </summary>
         public uint ParentID { get; set; }
 
+        /// <summary>
+        /// Are we sitting on an object?
+        /// </summary>
+        /// <remarks>A more readable way of testing presence sit status than ParentID == 0</remarks>
+        public bool IsSatOnObject { get { return ParentID != 0; } }
+
         /// <summary>
         /// If the avatar is sitting, the prim that it's sitting on.  If not sitting then null.
         /// </summary>
@@ -1808,6 +1814,8 @@ namespace OpenSim.Region.Framework.Scenes
                 SendAvatarDataToAllAgents();
                 m_requestedSitTargetID = 0;
 
+                part.RemoveSittingAvatar(UUID);
+
                 if (part != null)
                     part.ParentGroup.TriggerScriptChangedEvent(Changed.LINK);
             }
@@ -1887,7 +1895,7 @@ namespace OpenSim.Region.Framework.Scenes
                    )
                    ));
 
-//                m_log.DebugFormat("[SCENE PRESENCE]: {0} {1}", SitTargetisSet, SitTargetUnOccupied);
+            m_log.DebugFormat("[SCENE PRESENCE]: {0} {1}", SitTargetisSet, SitTargetUnOccupied);
 
             if (PhysicsActor != null)
                 m_sitAvatarHeight = PhysicsActor.Size.Z;
@@ -1920,6 +1928,12 @@ namespace OpenSim.Region.Framework.Scenes
                     AbsolutePosition = pos + new Vector3(0.0f, 0.0f, m_sitAvatarHeight);
                     canSit = true;
                 }
+//                else
+//                {
+//                    m_log.DebugFormat(
+//                        "[SCENE PRESENCE]: Ignoring sit request of {0} on {1} {2} because sit target is unset and outside 10m",
+//                        Name, part.Name, part.LocalId);
+//                }
             }
 
             if (canSit)
@@ -1930,6 +1944,8 @@ namespace OpenSim.Region.Framework.Scenes
                     RemoveFromPhysicalScene();
                 }
 
+                part.AddSittingAvatar(UUID);
+
                 cameraAtOffset = part.GetCameraAtOffset();
                 cameraEyeOffset = part.GetCameraEyeOffset();
                 forceMouselook = part.GetForceMouselook();
diff --git a/OpenSim/Tests/Common/Helpers/SceneHelpers.cs b/OpenSim/Tests/Common/Helpers/SceneHelpers.cs
index d5354cbcac..769de83c9c 100644
--- a/OpenSim/Tests/Common/Helpers/SceneHelpers.cs
+++ b/OpenSim/Tests/Common/Helpers/SceneHelpers.cs
@@ -412,26 +412,49 @@ namespace OpenSim.Tests.Common
         /// <returns></returns>
         public static AgentCircuitData GenerateAgentData(UUID agentId)
         {
-            string firstName = "testfirstname";
+            AgentCircuitData acd = GenerateCommonAgentData();
 
-            AgentCircuitData agentData = new AgentCircuitData();
-            agentData.AgentID = agentId;
-            agentData.firstname = firstName;
-            agentData.lastname = "testlastname";
+            acd.AgentID = agentId;
+            acd.firstname = "testfirstname";
+            acd.lastname = "testlastname";
+            acd.ServiceURLs = new Dictionary<string, object>();
+
+            return acd;
+        }
+
+        /// <summary>
+        /// Generate some standard agent connection data.
+        /// </summary>
+        /// <param name="agentId"></param>
+        /// <returns></returns>
+        public static AgentCircuitData GenerateAgentData(UserAccount ua)
+        {
+            AgentCircuitData acd = GenerateCommonAgentData();
+
+            acd.AgentID = ua.PrincipalID;
+            acd.firstname = ua.FirstName;
+            acd.lastname = ua.LastName;
+            acd.ServiceURLs = ua.ServiceURLs;
+
+            return acd;
+        }
+
+        private static AgentCircuitData GenerateCommonAgentData()
+        {
+            AgentCircuitData acd = new AgentCircuitData();
 
             // XXX: Sessions must be unique, otherwise one presence can overwrite another in NullPresenceData.
-            agentData.SessionID = UUID.Random();
-            agentData.SecureSessionID = UUID.Random();
+            acd.SessionID = UUID.Random();
+            acd.SecureSessionID = UUID.Random();
 
-            agentData.circuitcode = 123;
-            agentData.BaseFolder = UUID.Zero;
-            agentData.InventoryFolder = UUID.Zero;
-            agentData.startpos = Vector3.Zero;
-            agentData.CapsPath = "http://wibble.com";
-            agentData.ServiceURLs = new Dictionary<string, object>();
-            agentData.Appearance = new AvatarAppearance();
+            acd.circuitcode = 123;
+            acd.BaseFolder = UUID.Zero;
+            acd.InventoryFolder = UUID.Zero;
+            acd.startpos = Vector3.Zero;
+            acd.CapsPath = "http://wibble.com";
+            acd.Appearance = new AvatarAppearance();
 
-            return agentData;
+            return acd;
         }
 
         /// <summary>
@@ -440,6 +463,9 @@ namespace OpenSim.Tests.Common
         /// <remarks>
         /// This can be used for tests where there is only one region or where there are multiple non-neighbour regions
         /// and teleport doesn't take place.
+        ///
+        /// XXX: Use the version of this method that takes the UserAccount structure wherever possible - this will
+        /// make the agent circuit data (e.g. first, lastname) consistent with the user account data.
         /// </remarks>
         /// <param name="scene"></param>
         /// <param name="agentId"></param>
@@ -452,6 +478,10 @@ namespace OpenSim.Tests.Common
         /// <summary>
         /// Add a root agent where the details of the agent connection (apart from the id) are unimportant for the test
         /// </summary>
+        /// <remarks>
+        /// XXX: Use the version of this method that takes the UserAccount structure wherever possible - this will
+        /// make the agent circuit data (e.g. first, lastname) consistent with the user account data.
+        /// </remarks>
         /// <param name="scene"></param>
         /// <param name="agentId"></param>
         /// <param name="sceneManager"></param>
@@ -461,6 +491,17 @@ namespace OpenSim.Tests.Common
             return AddScenePresence(scene, GenerateAgentData(agentId), sceneManager);
         }
 
+        /// <summary>
+        /// Add a root agent.
+        /// </summary>
+        /// <param name="scene"></param>
+        /// <param name="ua"></param>
+        /// <returns></returns>
+        public static ScenePresence AddScenePresence(Scene scene, UserAccount ua)
+        {
+            return AddScenePresence(scene, GenerateAgentData(ua));
+        }
+
         /// <summary>
         /// Add a root agent.
         /// </summary>
diff --git a/OpenSim/Tests/Common/Helpers/UserAccountHelpers.cs b/OpenSim/Tests/Common/Helpers/UserAccountHelpers.cs
index 3d3e65c58c..2fbebc4ea9 100644
--- a/OpenSim/Tests/Common/Helpers/UserAccountHelpers.cs
+++ b/OpenSim/Tests/Common/Helpers/UserAccountHelpers.cs
@@ -138,6 +138,15 @@ namespace OpenSim.Tests.Common
             CreateUserWithInventory(scene, ua, pw);
             return ua;
         }
+
+        public static UserAccount CreateUserWithInventory(
+            Scene scene, string firstName, string lastName, int userId, string pw)
+        {
+            UserAccount ua
+                = new UserAccount(TestHelpers.ParseTail(userId)) { FirstName = firstName, LastName = lastName };
+            CreateUserWithInventory(scene, ua, pw);
+            return ua;
+        }
         
         public static void CreateUserWithInventory(Scene scene, UserAccount ua, string pw)
         {