Unbeknownst to them, they had been victims of "undercover karaoke."
The act had been filmed by FunnyOrDie.com, the comedy video website co-founded by Will Ferrell and Adam McKay. The video was posted Tuesday and was rapidly circulating online.
"I was a bit nervous to do this skit at first because I feared it would amount to little more than showing off some vocals, dressed as someone else," Jewel said in an e-mail. "I was worried it wouldn't be self-effacing enough."
Jewel walked onto the stage wearing a prosthetic nose, wig and glasses, and butt padding.
In the end, the singer-songwriter did reveal her true identity to the surprised audience.
The Associated Press contributed to this report.