One eyewitness said it "looked like" Gomez was lip syncing from the start of her performance and kept playing with her ear piece.
During what The Hollywood Reporter described as a "heavily backed rendition" of the track, "Undercover," Gomez swore into the mic.
"Sorry, the sound isn't working!" she told the crowd (as seen in this video). "Are you guys having a good night?"
"I promise, a lot cooler people are gonna come out tonight," she added. "Is it okay if I perform two more songs and you guys can get to it?"
"I have to thank you guys so much for allowing me to be able to perform the music that I love," Gomez said when her sound didn't improve. "And I have to say that before you guys go on to the people that you care about, I really appreciate you supporting everything that I do. So this next song is the first song I had hit No. 1, and it's called 'Come & Get It.'"
Gomez then left without finishing her last song.
2014: The Year in Music [PHOTOS]