Adam Lambert caused a major stir at Lady Gaga's 25th birthday party.

According to an exclusive report from Us Magazine, the 'American Idol' runner-up arrived drunk at the party, which he allegedly wasn't even invited to in the first place. (The report says he showed up as a guest of Gaga's tour opener, the Scissor Sisters.)

Perez Hilton claims that Lambert was "super sloppy embarrassingly drunk," and a source adds that "Adam acted like an animal. He kept jumping up on the tables and chairs and screaming. When they brought the cake out he tried to smear it in Gaga's face and put a doll from the cake in her mouth!"

In fact, things got so bad that Lambert was asked to leave by Gaga's security.

Lambert has since apologized for his behavior on Twitter, saying, "Was trying for celebratory gesture 4 gaga's B- pi–ata style...instead my fist is all bruised and la cita has a hole in the ceiling. #sloppy."

Talk about a 'Bad Romance.'

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