I'm in a semi-rant mode right now. I just finally watched Splice, a "creature-feature" thriller that came out last year and I want to punch the director in the face. The entire movie is a smart and emotional drama raising some very interesting questions, developing the "monster" into a sympathetic character... just to turn around 15 fucking minutes before the end, destroy the entire set up, destroy the developed character and turn the ending into a brainless "monster chasing heroes through the woods" routine for no apparent reason.
Do you remember any movies that were ruined by terrible awful third acts/endings?
"I Am Legend" with Will Smith also comes to mind. First two thirds of the movie were amazing - third act I wish I didn't see o_O