Continuing my internal series on keyboard short cuts, have to recommend Ctrl+Z this week.
This is known as the "Undo" or "Oh, shit!" shortcut. That is because it undoes the last thing you did.
This is mostly used in word processing or other programs where you make stuff. It is great if you deleted a chunk of material on accident, moved something where you didn't want it to go, or had an image go wonky in editing.
In many programs, you can keep Ctrl+Zing until you run out of things to undo.