There's nothing quite like a good film. Multiple acts of build-up centered on endearing an audience to characters, putting them through an arc that culminates in a satisfying conclusion that lets the viewers leave the theater with thoughts provoked and minds gratified.
One of the most effective methods of making a satisfying ending is the surprise twist. Done well, this can elevate the stakes of the third act by pointing out clear signs that the audience never picked up on culminating in a shocking reveal. But done wrong, this can tear down the suspension of disbelief by negating previous plot points, abruptly changing the tone of the movie, or being just too ludicrous to stomach.
While a weak or lame twist can be disappointing, it is downright tragic when the film was gearing itself up for a big, emotional ending that would have left one last dramatic impact on its audience. Seeing how a film could have had a potentially great finale but was ruined by a pathetic attempt to surprise the audience for no reason leaves the viewers more confused and upset over what could have been than satisfied with what they recieved.