9. Asteroids (Arcade) - High Score Shield
Granted, this one was in the arcade version of the game, and of course, the developers were quick to fix the glitch because it meant people were pumping less money into the machines, but for a short while, sly players could move their ship behind the score in the top left hand corner, and essentially become invincible. It was a quick way to get your name on the high score list, but Asteroid "purists" would certainly frown upon this, even if it proved a great way to kill lots of time without spending much cash. It improved the game because, while absolutely ridiculous, it allowed players to maintain a false of superiority when they saw their name in the high score list despite, in all honesty, not having earned it (though some will argue that finding out about the glitch before the age of the Internet could itself be considered earning it). And who doesn't love knowing something that their friends don't?