From Oswald's Mother:
As you can see by the diagram above, if there was a shooter on the grassy knoll, he didn’t make the kill shot. His angle would have made an exit wound on the left side of Kennedy’s head. Basically, ear to ear. As has been fully documented, Kennedy’s head wound was lateral, from the back to front, or front to back, depending on your orientation and beliefs. So if there were a second shooter he would have to be positioned at or near the railroad overpass in front of the motorcade. But that causes other problems too. If a shooter or shooters were positioned at the railroad overpass they don’t have a lateral angle either. In fact, a careful study of the angles will show that that the kill shot could only have been accomplished from behind.
I'm amazed (OK, not really) that people are still writing about this. For the last word on this subject, see this post.