When you try to make "incompatiable change" (such as changing class attributes, deleting methods, even deleting the whole class) in SE24, you get error message OO401 "You cannot perform this action in modification/enhancement mode".
NetWeaver ABAP based systems
OO401, You cannot perform this action in modification/enhancement mode, SE24, incompatiable change , KBA , BC-DWB-TOO-CLA , Class Builder , BC-DWB-CEX , Customer Extensions , How To
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