Kristen Stewart and Robert Pattinson are the poster couple in Hollywood, their on screen romance in Twilight blossomed to a real life romance, which had fans around the world screaming their names.

However, the couple have been rocked by a cheating scandal - Kristen Stewart's reported affair with her Snow White & the Huntsman director, Rupert Sanders.

"Kristen is absolutely devastated," says one source about the brief romance between the actress and her director Sanders. "It was a mistake and a complete lapse in judgement."

CELEBRITY CHEATERS

The hookup with Stewart's Snow White and the Huntsman director Rupert Sanders occurred recently, the source says.

"She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," says the source. "She never meant to hurt anyone. She's a good person who just made a bad choice."

A rep for Sanders, who is married and has two children, could not be reached for comment. Sanders wife, Liberty Ross, also played Stewart's on screen mum in the summer blockbuster, Snow White & The Hunstman.