Now they've got really, really, really bad blood.
Although it hasn't been released yet, Katy Perry registered a trademark for a new song called "1984," which people think might be a dig at Taylor Swift and her 1989 album.
As anyone who's been following world events for the past few years knows, the two pop stars have an ongoing rivalry. They have an ex in common (John Mayer), they've battled over back-up dancers, they've written songs (likely) about each other, and they've accused one another of mean girl tactics.
Watch out for the Regina George in sheep's clothing...
— KATY PERRY (@katyperry) September 9, 2014