Non-unique names in the Name Manager
Go to the Name Manager and click "New". Fill in the "Comment" and "Refers to" sections. Set the Name and Scope to a combination that already exists. Excel will complain and reload the New Name window, erasing everything that you had typed before. This can be quite punishing for such a simple mistake if your formula is long and complicated or if you optimized your comment to fit within the character limit. Excel should instead just return you to the New Name window that you had before.
Lord yes. Nothing should ever be destructive like that. Especially that tight little world.