Sarah Rorbach is another actor who managed to go on and find a degree of success after an uncredited background appearance in an early Harry Potter film. Can you see her hidden away in the picture on the left? The producers didn't even see fit to give her character a name, which you could understand if it was a one-off appearance, but Rorbach actually made three separate Harry Potter film appearances without once getting a formal acknowledgment. She's in "Sorcerer's Stone", "Chamber of Secrets" and "Goblet of Fire", playing the same silent Hogwarts student!
Rorbach has a small clutch of other minor roles and short film appearances to her name, and recently launched a new website where she advertises herself as both an actor and a singer, with over 20 years of stage experience to her name. Her agency website states that she's trained in physical combat, so Rorbach is one Harry Potter alumni you wouldn't want to mess with.