You know those games where you get to the final stage, and you have to fight all the bosses again, usually just before the final boss? Sometimes they're scattered throughout the stage, sometimes they're one immediately after the other. And sometimes instead of playing boss music, it just keeps playing the normal level music. I always liked that. I remember it being somewhat common, but I cant' think of too many off the top of my head.

The Megaman series
Bad Dudes
Metal Storm

I know there's a lot more than that, help me out here guys...