Thanks Kevin Cozens for a patch that:

Fixes several spelling mistakes

OpenSim/Framework/UndoStack.cs

@@ -90,7 +90,7 @@ namespace OpenSim.Framework
                 return deleted;
             }
             else
-                throw new InvalidOperationException("Cannot pop from emtpy stack");
+                throw new InvalidOperationException("Cannot pop from empty stack");
         }
 
         public T Peek()