These moments that change the plot from how it seemed it was going to go; these shocking revelations or unforeseen events that dramatically shift a story in a new direction. Done well, these moments can truly enhance a story and make it well-remembered for a very long time...
This, however, is not a thread about moments that enhanced the story; this is a thread about plot twists or other moments where the story took a sudden turn that you think the story would've been better off not having, and why you think it detracts from the story.
Note: since these moments are usually very important moments in a story, as in spoilers, please state the name of the story and then put the twist or turn itself in a spoiler tag.