Continuity mistake: Right after the bully has been killed by the motorcycle, in the wide shot, everything from his forehead to his chest is a totally mangled, disgusting mess. A little while later, though, in the close-up, his face, although bloody, is completely intact and recognisable.
Trivia: Towards the beginning of the movie when Drew is told to take Zowie the dog outside of Gus' house, there is a cross (crucifix) hanging on the wall to the right of the front door. Towards the end of the movie when the Vet comes to Gus' house, is attacked by Zowie, he enters Gus' house. Notice the cross is now hanging upside down, I'm sure to indicate evil is now resident there (although Gus was pretty much an evil bastard even when he was alive).