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.

SOURCE: http://www.people.com/people/article/0,,20614660,00.html