"Universal Undo"
Ever do something in Windows and think, "Oops! Wish I hadn't done that"? Try this:
Hold down the Control (Ctrl) key and press the letter "Z" (Ctrl-Z). 9 times out of 10, it will set you back to just before the "Oops".
Ctrl-Z is the "almost universal un-do".
Enjoy.
(See also: Keyboard_Shortcuts)