Question
AutoCAD elements not trimming or chamfering (with a not coplanar error message on the command prompt) or invalid hatch boundary or problems with measuring areas.
Answer
Check elements have any Delta Z values in properties (change to zero) Or alternatively use the Flatten tool that comes with the Express tools (this is not available for AutoCAD LT!) In any case the objects you are editing need to have the same 'z' elevation or height.