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If you delete an object in AutoCAD, keep working, and then you realize you shouldn't have deleted it and want it back, there is a command in AutoCAD that allows you to get that object back.
The OOPS command in AutoCAD will bring back the last deleted object, even after you kept working and doing more stuff. It has its limitations as you can imagine (for instance if during that "kept workin" you deleted something else) but still can save you from having to redo stuff.
This and more good stuff via How to undo almost anything in AutoCAD by Ellen Finkelstein