Selena Gomez's F-bomb trumps Miley's new hair

If you want to steer clear of Selena Gomez's bad side, don't "F" with her audio.

The 21-year-old singer let the "F"-bomb slip during her performance at the 2013 Jingle Ball in Los Angeles Friday, as she was apparently aggravated by a faulty sound system.

Eyewitnesses have reported to E! and Us Weekly that they could clearly hear her say "what the f***?" loud and clear.

Gomez reportedly apologized afterward, explaining to the crowd that the sound wasn't working and offering to perform a few more songs. However, reports suggest that she ended up deciding to just leave the stage early.

Meanwhile, Miley Cyrus twerked on a Santa and debuted a new, bobbed hairstyle - but, for once, it's not the "Bangerz" star who stole the show.

[youtube=http://www.youtube.com/watch?v=CUvtA4pr9cs&w=450&h=315]