What movies have a disastrous last act that ruined it for you despite being otherwise excellent?

This could be things like: a sloppy twist, a poorly executed genre shift, a new character out of nowhere, abrupt pacing changes towards the end, an unearned Deus ex Machina, or many other things.

For me the most memorable example is Now You See Me. It was a fun romp until the end when it goes full melodrama reveals.