What are some movies that you like that you felt were derailed by their ending or final act?

The idea for this thread came to me while watching the film, Collateral, by Michael Mann.

The film started off great. The cinematography was incredible, the performances by Cruise, Foxx, and Ruffalo were phenomenal, and the movie delved deep into which each main character was coming from in a manner that I thought was profound.

However, it all turns to **** during the third act, wherein ( spoilers) it is revealed that Foxx's previous client happens to be the next person Tom Cruise is assigned to kill. This revaluation has got to be one of the stupidest plot twists I have ever encountered in a movie.