Actually, I don't think the Mario thing with 99 lives is a glitch. It happens when you get all the stars, and Yoshi appears on top of the castle and gives you that many lives.
But anyway, back to the topic, there was a funny thing which happened to me with a glitch. I'm not sure if it counts, but me and my friend were once playing Ratchet and Clank, and his Dad was intrigued, and we let him have a go. Somehow he managed to jump over one of the energy barriers, which was weird, considering you're not meant to access that area at that time. The funny thing was he didn't understand what was happening either. It was a good day. :D