At another forum there were a discussion about a legendary glitch in Super Mario Bros, "unlocking" 256 new "levels" in the game. Most people dismissed it as a rumor, without even trying it. Now it is however proven to work. 
I just wanted to share it with you guys, now I have to get a copy of Tennis 
Edit: the information from the clip:
Turn on your consol with Super Mario Bros.
After a while, pull out the cartridge without turning the power off!
Then put in a Tennis cartridge and press reset.
Then play Tennis for a while.
Then pull out Tennis without turning the power off, and put in Super Mario Bros once again.
Then push reset, and then press A + START on the joypad.
Now something really strange will happen! :D
Beware, I live!
I am Sinistar!
Beware, coward!
I hunger!
Roaaaaaaaaaar!
