"For games that have multiple difficulty levels, like fighting games and shooters, then multiple endings per level are okay."

Ya. In fact, you could say that they are good. I remember beating Where's Waldo for the NES on "hard" for the first time as a kid, and i got so pissed that it gave me the same crappy ending as "easy". But that's a crappy game anyway- waldo's like a chameleon in that game because his sprite always changes colour, usually blending in with other people and objects around him. You'd have to play the game a million times to figure out which one's are his sprites when looking for him. Anyway, i wanted to add to the example.