Sometimes I miss the obvious. The Zapruder film contradicts the autopsy photos and even itself. The massive forehead wound is not present in the autopsy photos which show a wound to the top of the head (this does not mean the autopsy photos are legit either). Frame 374 shows that part of the back of JFK's skull is missing while other frames show the rear of his head intact.
What to make of this? Obviously the Zapruder film was doctored make the forehead wound to appear to be an exit wound and to conceal the exit wound at the back of JFK's head.
This isn't the only argument against the Z. film's authenticity but it's certainly a very easy one to grasp.