Kristen Stewart cheated on Robert Pattinson with 'Snow White' director Rupert Sanders - report

Hold on to your seats, "Twilight" fans. According to a report from People magazine, Kristen Stewart cheated on long-time boyfriend Robert Pattinson with her "Snow White and the Huntsman" director Rupert Sanders.

The bombshell came in an anonymously-sourced item posted Tuesday (July 24) to the magazine's website.

"Kristen is absolutely devastated," a source tells People. "It was a mistake and a complete lapse in judgment."

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

Neither Sanders nor Stewart have commented on the report. Obviously.

Stewart and Pattinson recently appeared together at Comic-Con to hype the final installment of the "Twilight" saga, "Breaking Dawn - Part 2."

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UPDATE: Us Weekly posted a copy of their new cover, which includes a purported picture of Stewart with Sanders and details on how the alleged liaison unfolded. Us claims the pair was caught together -- at least on camera -- on July 17 in a "series of steamy rendezvous."

Us also adds the detail that Sanders is engaged to model Liberty Ross, with whom he has two young children.