Fist II for the C64... sometimes you would get stuck in a certain area of the game and not be able to get out of it. The screen just wouldn't scroll away. Also, I'm not sure if it is a bug or not, but sometimes you'd end up stuck in one spot, endlessly fighting enemies. I don't know if they ever stopped coming.
How about games *enhanced* by bugs? I found a bug in Mastertronic's Ninja for the C64, where you could leave the bounds of the map and enter rooms based on whatever was outside of normal map memory.
To do it, you would find a place where your back is facing the wall, and then jump straight up. While in the air, hold down+back (angle). When your ninja lands, he'll twitch for a moment and then walk backwards, right through the wall! At this point you're in Never Never Land. The backgrounds are all screwy, and so are some are the enemies. It's very challenging as well... some enemies have messed up animations, and end up holding their kicking foot out or kicking very rapidly, which will decimate your ninja in no time.
Great fun.
