Director Peter Berg tells the story in the latest issue of Details magazine:

We went to a Beastie Boys concert on New Year’s Eve—maybe ’89. He was totally sober. Didn’t drink. But he decided he wanted to stage dive. He jumped up on stage—and nobody caught him. He landed fucking flat on the concrete. There were these huge Samoan security guards—one of them picked him up, and another hit him. Ari grabbed the guy by the back of the head, bit down on his face, and pulled a big piece of his cheek off. They were going to kill him. But the Samoan with the bloody face looked at Ari and said, “You’re crazier than me. I like that. Now get the fuck out of here.” That’s a true story.