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

ahhh, back in the good old days when you had to manually clear out ram
. quite a few early genesis games do weird stuff with cart swapping like this as well,
regardless that is pretty neat

I've never heard of this glitch. That's actually pretty awesome.

Why'd he stretch a 4:3 image to 16:9. :(

umm. no. Those are not levels, just broken ish.

He was landing on things that weren't even there.

He's not, he's swimming

lol. your right, my bad.

Nice. Broken levels are always fun, like Glitch City from Pokemon Red/Blue

9-1 freezes right when the video ended though, I've gotten to 9-1 by modding where the warp pipes send you

... cool and simultaneously strange...

Wow, I play my NES on my HDTV too! Pretty awesuum glitch! :3