Glitch

A Glitch is usually a program error or a problem in a game. It usually happens when something in a game is done in a different way than the programmers thought. Usually, glitches are normally harmless, but some kinds can delete stored data. Usually, when a glitch is done, the screen may turn blank or different colors will pop up, strange sounds appear, or the game will freeze or crash.

Examples.
One of the most famous glitches is the "MissingNo. Glitch" from the original Pokemon games.

In Animal Crossing.
There are very few glitches in the Animal Crossing series. Pretty much none of them are harmful. One of them, as demonstrated in a posted video, is harmless and can actually make a players house look better.

MissingNo. Glitch: http://www.youtube.com/watch?v=dr9xlgH-dkU

Outside a house's barrier: http://www.youtube.com/watch?v=11m1L_13nqg