Bug: This glitch has been fixed in later versions of the game, but still works in the original NES version. When fighting the Yellow Devil, select your Elecbeam and fire it at the enemy's eye. As soon as you fire, start rapidly mashing the Select button over and over again as fast as you can till it hits. If done fast enough, the beam will instantly kill Yellow Devil in one hit.
Trivia: The promotional commercials in Japan for this game show Mega Man using the wrong weapons against the Master Bots. Like using Thunder Beam on Guts Man, who is weak to Hyper Bomb. Using Rolling Cutter against Fireman who is weak to Ice Slasher. And Fire Storm against Ice Man who is weak to Thunder Beam.