The Map Editor remembers the last action you performed during the current session. If you make a mistake or change your mind, the Undo command in the Edit menu can usually reverse the action. It can restore a control or other item if your last action was to delete it, move, size, or align it.
Operations that you cannot undo are:
In 4.620 there is only one level of Undo, and there is no Redo facility. Previous versions of Mapedit had a very limited undo facility - for example there was no undo for move, size, or align operations.
If you find you cannot undo a serious mistake then close the map without saving. It is a good idea to save your work periodically when creating a map. You may be able to use the Copy and Paste facility to recover from certain mistakes that undo cannot reverse.