Talk about bad blood!

Taylor Swift might have thought that she was going to have a drama-free night celebrating bestie Gigi Hadid's birthday at Nice Guy yesterday evening... but then John Mayer showed up!

In case you forgot, T-Swizz basically hates the notorious lothario because they used to date before he cast her aside once he sealed the deal. She even wrote her song "Dear John" about how much of a sh*thead he turned out to be! What was ol' Johnny boy even doing there? He's 38-years-old and hitting up a 21-year-old's party. He needs to get a life and stop trolling for a young chick to make his next victim.

We bet Tay Tay wishes she had brought boyfriend Calvin Harris with her!