Ashley Simpson was the musical guest. When she sang her first song you could tell she was lipsyncing. Then, when it was time for her to play again, her first song started to play again and the vocals started but she was just standing "not singing". So she runs off the stage and they cut to commercial.
When the show ended she was out with the cast and blamed it all on her band.
I thought it was hilarious watching her get made for lipsyncing on live TV.