Input the password 1415 5585 7823 6251 and hold L+R while pressing Start to confirm it. This will take you to the Versus Mode, a two-player mode.
This will pit Mega Man against Bass (or Mega Man against Mega Man or Bass against Bass) in a single-screen stage with four platforms. Both characters play exactly the same way. You can only fire one shot at a time, and that shot must leave the screen before another shot is fired. You can charge your shots, but only fully. There's no medium strength charged shot. You also can't slide. Dealing enough damage quickly enough will cause your foe to fall down for a moment. They can also see tiny Beats floating around their heads, indicating that they are stunned.
The first player to completely diminish their foe's health wins the round, and the first two win two rounds wins the match. This really is just a little extra easter egg mode.