According to the #100 issue of Nintendo Power, you could have Mega Man fight Bass in a Street Fighter-style match. You would enter:
1415
5585
7823
6251
as your password. While holding L and R, press Start.
If you didn't hold L and R, you would enter a normal game with every item and weapon.

I tried this code on Megaman Anniversary Collection, and it doesn't work; it just gives me the normal game with every item and weapon. Did they exclude this extra?

you need to press y and start
if that doesn't work, try y and x then start.

P.S.You nee d to press them at the same time, or you canpress y or y and x first and hold it and press start.