The two actors accidentally struck each other with the cameras rolling, one or both of them became enraged, and both ended up fighting each other. This resulted in the accidental filming of a fight, with real wounds, real lost teeth, and real blood all over their clothing.
Here it is in all its uncut glory. You'll never see another fight scene like this again.
I hit that wall so hard, man. That was such a tough fight scene. We didn't use any stunt doubles at all. He broke three of my ribs and I busted his nose. ...After he busted three of my ribs, I hit him with a bottle, a real one. ...He busted three of my goddamn ribs and I couldn't even breath and he was still hitting me.
When he whacked me with that board, he missed the knee pad and hit me right there [indicating a spot just below the knee]. To this day when I talk to him, I accuse him of doing it on purpose. Luckily that was the last take of the whole movie.