I was playing Super Mario Bros. on my NES. When I got to level 6-3, I killed Lakitu and jumped up and died. I thought I had discovered a glitch. As it turned out, it wasn't, but I did learn something about Lakitu's movement. If you get a power-up while Lakitu is on screen, he automatically begins to throw a spiny shell, which can make Lakitu flash if the game doesn't "expect" it. He breaks from his usual shell throwing pattern when you get a power-up is what I'm saying. This is most noticeable on level 4-1. Just thought it was interesting. Not necessarily a glitch, but I thought it was kind of interesting.