One of the finest features to ever be added to software is the UNDO command.  This includes AutoCAD when it was added in AutoCAD Release 2.5 (mid 1980s) because apparently users prior to that never ever made a mistake.

Funny AutoCAD history tidbit: Before the UNDO command was added into AutoCAD you had to close the drawing without saving to be able to undo what you last did.

Cheers,
Shaan