In Mega Man 6 for the NES, “all of the acquired robot master weapons use the same sound effect as an uncharged shot with the exception of the Centaur Flash and the Plant Barrier.” ScrewAttack takes a look back at this title after the break.
As with Mega Man 5, the same musical theme for the fortress stages was used for four consecutive levels, as opposed to using a theme for two levels and then using a different one.
[via Wiki]