If you're lucky enough to have a copy of Final Fight 2, you know that you (and optionally a friend) each choose one of three characters to wander through Metro City and beat up bad guys. Pretty standard stuff.

But, if you go to the title screen

And press

Down, Down, Up, Up, Right, Left, Right, Left, L, R

On Controller 1, the background will change from Black to Blue.

Signaling that the code worked. This will allow you to choose the same character for both players

You know, if you're into that kind of thing.