No XYZZY?
This cheat appears in many old-time games. You will know that it work if the game responds with "nothing happens" and nothing happens.
However, in some games (minesweeper for example, if you press right shift after you enetr the code, the top right/left pixel of your screen will change color over a bomb), it is broken, and something happens when you type it.
Here's some history for you hooligans:
http://www.rickadams.org/adventure/c_xyzzy.html