"You think you've heard LOUD screaming in your life. Then @justinbieber comes out and does a surprise song during your show. Woah. Unreal," Taylor Swift tweeted last night after concluding a show at the Staples Center in Los Angeles in which the teen heartthrob turned up to perform his massive hit "Baby." As you can see in this fan-shot footage from the gig, pretty much everyone in the house was flipping out for Bieber, including Swift herself, who seems overjoyed by his presence.