Bridge to Terabithia.

As a little boy, I didn't really understand the nuances of good storytelling. But when a movie builds up two main characters, a guy and a girl, and makes the girl considerably more likeable than a guy, but then about 3/4 of the way through the movie she dies offscreen, even I knew that it was a bad story. Not only that, but the main dude in the movie literally forgets about the girl that showed him this magical world, and spends the last five minutes in Terabithia with his sister, having apparently easily replaced the first girl.

First time I shed bitter tears at a movie and knew, in my brain if not my heart, that the movie had not earned it.