Danganronpa: The Animation
For a mystery thriller, Saw type anime, I thought it was good and worth a watch. But the ending went completely against what the way the whole series was set.
For those who haven't watched it (Non-Spoiler Synopsis): Essentially 15 students are trapped in a school by an evil bear (like Jigsaw from Saw). In order for a student to be free, a student must kill another one of the 15 students and must go through a court case with the other students and not be found as the killer, if the students choose someone else as the killer they are all killed and the killer is free, but if killer is found guilty just the killer is killed. The clues were actually set up for the viewer to make the assumption on who was the killer was and everything made sense.
For an anime that allowed clues to have the viewer figure out what happened, this really came out of no where and made no sense at all. It was not set up (or atleast not well), went completely against what was done, exposed some of the show's big flaws, and even for the people I know who really liked it hated the ending.