Quote from Brass:
Again, and just for the sake of argument and nothing else, the defendant could have walked up to a building wall and slammed the back of his head against it a couple of times. Just enough to draw blood, which is rather easy to do with the scalp. Of course, he would have had to erase any traces of blood off the wall. But then, was there any blood evidence from Zimmerman's head on the concrete he alleges his head was banged against. We, on the outside, don't know. As I recall from the eyewitness account, the person on top, who got up after the gun shot, walked towards the general direction of the witness, who was inside her apartment looking out the window. Not specifically to her, but in that general direction. Towards a building.
Again, I'm not suggesting Zimmerman did any such thing. Rather, we are discussing conceivable possibilities solely for the sake of argument.