The hookup was allegedly with Stewart's “Snow White and the Huntsman” director Rupert Sanders. "She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened," said the source. "She never meant to hurt anyone. She's a good person who just made a bad choice. Kristen is absolutely devastated. It was a mistake and a complete lapse in judgment."
Stewart, 22, and Pattinson, 26, were recently together at Comic-Con 2012. The two have been mostly coy about their romance, but recently showed more affection in public.
Actress Kristen Stewart reportedly had a brief fling with a married director, a source told People.com, jeopardizing her relationship with her “Twilight” co-star Robert Pattinson.