the parameters for llAtan2 were inverted before passing to Math.

Thanks to Rob Smart for pointing this out.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs

@@ -344,7 +344,7 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
         public LSL_Float llAtan2(double x, double y)
         {
             m_host.AddScriptLPS(1);
-            return (double)Math.Atan2(y, x);
+            return (double)Math.Atan2(x, y);
         }
 
         public LSL_Float llSqrt(double f)