Mentioned already, but KotORII has a pretty awful ending. Sure, there's like, one or two cool tidbits like Mandalore = Canderous But it's almost as if the developers were celebrating "finishing" (term used VERY loosely) the game when one of them says, "Oh shit, we forgot to put an ending in," so they run back to their computers and quickly hash together a couple of paragraphs of text, then call the final boss' voice actor up to quickly run through the last remaining lines.
In a movie, I thought that Ocean's Twelve was terrible. The "twist" at the end made me genuinely peeved to have spent money to watch the entire thing, and the movie was actually pretty bad to begin with.