Mega Man 3 (USA).nes: 26F39 = 00
You can find out what that does for yourself.
comments
The address assumes a ROM that still has the 16-byte header. It's a ROM, after all.
I'm not spoiling what the surprise is — you have to see what it does yourself!
I'll just say that it took longer than I had expected to find the right byte to modify. The other ones would have been a lot easier...

