If you have ever been dragging a bunch of files from one location on the computer to another (say, from the desktop to a folder) with the mouse and accidentally dropped them in the wrong spot, you can reverse the move. The Undo command on both Windows and Mac OS X works here as well - as long as you use it immediately after the accidental action. Just go to the Edit menu on either system and choose the Undo Move option. You can also use the popular Undo shortcut on the keyboard, Ctrl + Z in Windows or Command-Z on the Mac.
