gametrailers.com

8yAK TIL The original Super Mario Bros game had 256 “Hidden” levels that were only accessible by swapping the game cartridge with a copy of “Tennis” while the game was running.
glk8 TIL That Michael Jackson worked on the music for Sonic the Hedgehog 3, but quit either because of his child molestation scandal that was going on, or that he was disappointed with the sound of the Sega Genesis. Some samples of his songs still remain in the game, though.
WNXA TIL that the original Super Mario Brothers game actually had 256 “hidden” glitch-filled levels that could be accessed by swapping out your cartridge with “Tennis” without turning the console off