Control Z is probably the most important control key of all. You will find that familiarizing yourself with this function will save you a considerable amount of headaches when you know that you have gotten ahead of yourself and want to return to or correct an idea.

"Control Z" or "Undo" is in the "Edit" menu.

Wouldn't it be nice if we had a similar command in our life menu? It could let us "Undo" most of our mistakes.