If you're going to insult Katy Perry, expect some serious shade in return.
A quick primer: Yesterday, Taylor Swift's Rolling Stone profile found her discussing a new song off 1989 called "Bad Blood," which is about an unnamed female celebrity who was, you know, "Mean," and also professionally screwed her over. The Internet quickly decided said anonymous artist was Perry -- some of Swift's dancers left her tour to go to Perry's, they've both dated John Mayer, and other 'evidence' points.