I'm old enough to remember when glitches were a fun thing to do after you'd beaten the game. The best example I can remember is in the original double dragon there's an area near the beginning of the game with two drainpipes, a barrel, and enemies. If you did it just right, you could throw the barrel at the enemy and knock them both into the drainpipe, causing both the barrel and the enemy to slowly levitate up and off the screen.

It sounds dumb, but in 1989-ish, I remember my brother trying to do that for HOURS, while the rest of us watched. We all laughed like idiots when he finally did it