Son of a Glitch!

[~Axis~]

I've noticed that many games have glitches and that some of them can be repeated and are rather amusing. What glitches have you guys found? I remember quite some time ago as a kid that my cousins and I used to play Super Mario Bros. and there was this glitch where in world 1-2 near the end at the pipe as big mario, you jump on the pipe and without breaking the block at the very edge, you duck and jump backwards "into" it and it pushes you into the extra world selection area except it just barely shows you a single pipe. You jump into it and it sends you to world -1! That level is impossible to beat for some reason.