Mega Man III second-controller uber-cheat
Mega Man III had a really nifty cheat mode that i've never seen anything like. It was so subtle-- so obvious, so right out there in plain view, and yet there was no way you'd ever find it by yourself.
There were two key combinations on the second controller that would do special things. right and a-up.
Holding down right would give you an unbelievably high jump-- like, you'd touch the top of the screen. The really nifty bit is that if you fell into a hole with this in effect, the hole wouldn't hurt you-- you'd just be walking around in the hole. If you jumped into the hole and held the jump button down with this in effect, you'd bounce up and down crazily. This was the only way some people could beat the Magnet Man stage.
There is a weird bug related to this triggered by a specific series of events. You have to find a flying enemy and a hole. Jump such that you hit the enemy, get hurt, and while sparking and everything fall into the hole. The Mega Man Death Sound will go off, the music will cease and your health bar would go down to zero. At that exact moment, start holding down right, and then jump back out of the hole. Your health will still be at zero. Your health will still be at zero no matter what happens to you. And so until you pick up a life container and break the spell, you would be invincible, and there would be no music. Of course, you'd have by this point probably wasted four lives trying to make the trick work and failing, so it really wasn't worth it in a serious game, but it was cool to see.
Holding down a and up at the same time was the weird one-- it was widely referred to as the Freeze code, but it didn't actually do that. It usually did something that was sort of freezing, but what it did varied wildly from enemy to enemy, and almost nothing actually froze. Almost all enemies would stop changing animation frames-- like even if they didn't stop moving around, they would freeze within their internal animation. Some of them would still move, but they would lose the ability to jump. Some of them would move for a second more and then stop. Some things would stop shooting at you while the freeze code was in effect; some wouldn't. It helped in some places, but in others it just gave you a false sense of security. I think Mega Man couldn't walk normally while the freeze code was on, but i don't remember.
(Proj2501, who also corrected some major factual errors in the original version of this writeup, would like you to know that Needle Man was the enemy most susceptible to the freeze code.)
So even though Mega Man is one-player, Mega Man III became a group activity-- we would get one person to play, and the rest of us would trade off with the all-important second controller, allowing one person to manipulate the enemies and the jump while the other devoted his thought to playing. You can call this cheapness, but Mega Man as a communal, shared activity is a great deal more meaningful than as a one-player thing. Oh, and doing the freeze code without a second person was literally impossible, but a lot of the time you'd see someone sitting there playing, with the second controller on the floor in front of them, activating the jump code with their toe.