Okay so this is a little cringy.
Christina Aguilera recently stopped at a bar in New Orleans with a couple of her back-up dancers during an off-night of her Liberation tour. There was a local reggae band, Claude Bryant and the All Stars, playing and Christina went up and asked the band if she could join them and sing. The band politely declined her offer, and sent Christina on her way.
After being interviewed by a local news channel the next day, the band said they didn't recognize, but would've gladly let her on stage had they know it was the superstar. Xtina even talked about the whole situation at her show the next night. Luckily, someone at the bar caught the whole thing on camera: