Mayer notoriously romanced Swift, who wrote "Dear John" about him, in 2010 and had and on-again, off-again relationship with Perry from 2012 to 2014. The claws came out in September 2014 when Swift revealed to Rolling Stone that her new song "Bad Blood" is about a rival female singer who is "very competitive." "For years, I was never sure if we were friends or not," Swift said. "[Then last year] she did something so horrible. I was like, 'Oh, we're just straight-up enemies.'" Perry stoked the fire with some Mean Girls shade.

After Vampire Diaries co-stars Somerhalder and Dobrev split in May 2013, she dated Hough, brother of her pal Julianne Hough, for six weeks in the fall (and even spent the holidays with the Houghs). By spring 2014, the Dancing with the Stars pro had a brief romance with Dobrev's close friend Reed, who started dating Somerhalder in the summer. Making this vampire quadrangle even more complicated? Somerhalder was linked to Reed's Twilight co-star Ashley Greene in 2009. Guess he has a type?

These Friends are so close that they even dated the same guy! The Counting Crows frontman dated Aniston in 1995 before hooking up with Cox. Of his romance with the former, Duritz revealed, "We never even slept together." But his relationship with Cox was far more serious: Duritz wrote the song "Monkey" about her and she appeared in the band's music video for "A Long December."

Shortly after Locklear filed for divorce from Sambora in February 2006, Richards started dating the Bon Jovi rocker, but she insists there was nothing unsavory about how they got together. "I was no longer friends with Heather months before Richie and I got together. I don't want to say what caused the split, but she and I weren't even speaking then," she told Redbook. "Yes, I got together with Heather's soon-to-be ex-husband, but no, I didn't have an affair with him. If Heather and I had been friends at that time, I would never have crossed that line. But we weren't. The friendship was not salvageable." The couple split in 2007 and dated again from 2011-12.

Sounds like something straight from Wisteria Lane. Longoria and Kay, who's now married to Covert Affairs star Piper Perabo, dated in 2004 after he directed her in the TV move The Dead Will Tell. Two years later, Kay dated Hatcher for 10 months after meeting through a mutual friend, who, ironically, was not her Desperate Housewives co-star. At the time, Longoria's rep said in a statement, "Eva and Tony [Parker] have never been happier together, and Eva wishes the same happiness for Teri."

Swift, who's pals with Lovato and Greene, was infamously dumped by the JoBro in a 27-second phone call in 2008 (after which she immortalized their relationship in the tune "Forever & Always"). Jonas and Lovato dated for a few months in 2010, but did not break up over the phone, before he moved on to Greene, whom he dated for a year. In a first-person piece for Vulture in 2013, Jonas revealed that he lost his virginity at 20, which was when he started dating Greene.

After Taylor's third husband, producer Mike Todd, died in a plane crash in 1958, she turned to his best friend, Fisher, who was married to her good friend Reynolds, aka "America's Sweetheart." The affair was a huge scandal and led to the cancellation of Fisher's NBC variety series. The pair wed days after his divorce from Reynolds was finalized in 1959, but split five years later. Reynolds later forgave Taylor, even co-starring in These Old Broads with her, and they remained friends until Taylor's death in 2011.

Carter and Duff started dating when they were 13 in 2000 until he left her for Lilo a year and a half later. "I got a little bored so I went and I started getting to know Lindsay, dating Lindsay," Carter said in 2005. And boy, does he regret it. Duff, who has a son with estranged husband Mike Comrie, remains the one who got away for Carter, who continues to declare his love for her. Duff finds it all "weird." Take a hint, Aaron. She's just a bird that's already flown away.

Shania Twain, Marie-Anne Thiebaud, Robert "Mutt" Lange, Frederic Thiebaud The country star's 14-year marriage to her producer Lange ended in 2008 after he allegedly had an affair with one of her best friends and their personal assistant Marie-Anne Thiebaud. Twain and Marie-Anne's husband, Frederic, sought solace in one another and got married in 2011. "Frederic Nicolas Thiebaud has been a true gift to me as a compassionate, understanding friend and over time, an amazing love has blossomed from this precious friendship," Twain wrote on her website.

Boyd and Harrison, who penned "Something" about the model, wed in 1966 after meeting on the set of A Hard Day's Night. Clapton, who frequently stayed at the couple's home, became infatuated with Boyd and wrote "Layla" about her in 1970. Boyd and Harrison divorced in 1974, and she married Clapton in 1979. They divorced 10 years later after he fathered two children with two women.

Endless Love? More like endless string of co-star romances. Pettyfer dated Roberts in 2007 after meeting on the set of Wild Child and then romanced her pal Agron in 2010. They broke up a day after their film I Am Number Four came out in 2011. Pettyfer later dated his Magic Mike co-star Riley Keough.

Three days before her wedding to Jack Bauer in June 1991, Roberts called the whole thing off, and on the day of, she jetted to Ireland with his pal and Lost Boys co-star Patric. In 2006, Sutherland told Rolling Stone that he didn't hold any grudges against his ex-fiancée. "I commend Julia for seeing how young and silly we were, even at the last minute, even as painful and as difficult as it was," he said. But he hadn't forgiven Patric yet. "It's not a matter of that. We were friends, and I’m surprised that I never got a call from him saying I’ve fallen in love with da-da-da," he said. "Instead, I found out from a stranger." In 2011, however, Sutherland and Patric co-starred in the Broadway revival of Patric's father Jason Miller's play That Championship Season.

Follow us here. Not only are Romo and Crawford friends, but they're brothers-in-law! The Dallas Cowboys quarterback dated Underwood in 2007 and "did [her] wrong." Shortly after, the American Idol champ went out with Crawford for a few months. In 2009, after dumping Jessica Simpson a day before her birthday, Romo started dating the Gossip Girl star's sister, beauty queen Candice Crawford, and married her in 2011. Underwood has moved on as well with hockey player Mike Fisher, whom she married in 2010 — on Simpson's birthday.