Kristen Stewart Cheats on Robert Pattinson with Married Director: Sources

The relationship between Kristen Stewart and Robert Pattinson has been rocked by the revelation that the actress had a brief fling with a married director, sources tell PEOPLE. "Kristen is absolutely devastated," says one source. "It was a mistake and a complete lapse in judgment." 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. A rep for Stewart had no comment. Stewart, 22, and Pattinson, 26, were all laughs just days ago at Comic-Con, even joking about their on-camera smooching scenes. The pair met on the first Twilight set and reports began almost immediately that they were a a couple.After being initially coy, they have become increasingly open about their relationship.