|Kristen Stewart & Robert Pattinson|
The 23-year-old actress was left red-faced when she and boyfriend Robert Pattinson showed up to watch hip-hop act Jurassic 5 at the Californian music festival, only to discover the model - whose husband Rupert Sanders had an affair with Kristen last year - was standing nearby.
An eyewitness told The Sun newspaper: "Kristen has tried so hard to put everything from last year behind her - and she still feels incredibly guilty.
"She and Rob had turned a massive corner in recent months and they were both there to enjoy themselves.
"The last thing she wanted to do was make Liberty feel uncomfortable, either. She looked pretty upset in the backstage bar later on that evening."
Liberty, 34, filed for divorce from her husband of 10 years, Rupert, in January after his "momentary indiscretion" with the 'Twilight' starlet. However, she appeared unconcerned by Kristen's close proximity at the festival and was accompanied by her new boyfriend Jimmy Iovine and his friend, U2 singer Bono.
Meanwhile, Robert was more preoccupied with meeting his musical idol Damon Albarn than his girlfriend's past. The actor introduced himself to the Blur frontman backstage and the pair chatted about British music over a couple of beers.