I used to have a lot of fun with my Game Genie back in the old NES days, and part of the reason that was fun was that not all of the effects were necessarily cheats. There were a lot of "unofficial" codes that would work perfectly well, although how they altered the game didn't necessarily mean you could play it normally. You could input a code for Super Mario Bros that would only let you play one level that looped forever, but it was a weird rendition of a normal level. One code for SMB3 wouldn't let you actually play the game, but it would cycle through the entire soundtrack.
Some of them would have been pretty game breaking if you could actually complete the game with them, but their effects prevented that. SMB2: all enemies commit suicide when they see you, but you can't get past the first world because the boss takes the key with him when he dies.
It's rather funny otherwise.