Taylor Swift loves bringing surprise guests on stage with her, but the latest one might also be the most unusual. Swift welcomed her "old friend" Nelly to the tour, and he busted out the big guns with a rendition of his 2002 hit "Hot in Herre." Naturally, Swift and her posse, Haim, got their synchronized groove on while he rapped. It's everything you never knew you wanted. Swift even posted videos (below), and you can check out all the pictures!