11. Black-balling Cho - The Order Of The Phoenix
Cho Chang was very mistreated during the film series. Reduced to someone for Harry to fawn over during Goblet of Fire and Order of the Phoenix, then practically disappearing from the rest of the series.
It was in the fifth film that the character was given a big injustice. She begins a nice little romance with Harry that she feels guilty about. Then when Dolores Umbridge goes on her rampage, practically reducing the school to a military state, she discovers the Room of Requirement that the DA was using as a classroom. It's revealed that Cho was the one to blab their location and, despite the fact it was clear she was forced to do so, Harry totally ignores her for the rest of the film.
It was later revealed that it was down to the Veritaserum (a truth potion) that Cho revealed the location, and yet she's still ignored / forgotten about by Harry and the gang. Story-wise, she served her purpose and the writers obviously had nothing more to do for her so she was relegated to background character.