Christina Aguilera was at a bar in New Orleans with some of her backup dancers and there was a dude on stage singing so Christina thought it would be cool to jump on stage and try to sing with him. The guy totally denied her and wasn't willing on giving up the mic. She told the story at her show the next night. When the guy found out about it, he said he didn't recognize it was her and that she's welcome back to sing with him anytime. More details here