What's The Most Frustrating Or Annoying Unexplained Plot Twist On A TV Show Or Movie?

When played right, plot twists make for an exciting, edge-of-your-seat story, and uncovering how they happened is part of the fun. However, sometimes, when you're watching a TV show or a movie (especially if it's a sequel), they throw in an unexpected plot twist for shock value and then just never explain it.