I think that perhaps the most compelling physical impossibility in the Zapruder film is the split-second slowing of the limousine in Z294-304. This event is noteworthy because it has been detected, measured, and discussed by scientists, including Dr. Luis Alvarez. From Z295-304, the limousine slows from 12 mph to 8 mph, a deceleration of about 0.37 g, in the space of barely half a second. Said Alvarez,
"The heavy car decelerated suddenly for about 0.5 sec. (10 frames), centered at about frame 299, reducing its speed from about 12 mph to about 8 mph" (American Journal of Physics, 44:9, September 1976, p. 825).
But Alvarez did not grasp, or chose to ignore, the implications of this rapid slowdown for the film's authenticity. Going from 12 mph to 8 mph in 10 frames would have produced a deceleration of about 0.37 g. As other scientists have noted, a deceleration of 0.37 g would toss things around, especially the occupants. Most cars do not decelerate more than 0.4 g. Yet, in the current Zapruder film, we see no visible effect on the occupants in the limo from the deceleration. The fact that JFK is not moved by this deceleration is particularly interesting because he no longer had voluntary muscle control and should have been thrown forward. Yet for many frames before and after this event, he appears to be quite immobile.
I suspect that this split-second rapid slowdown is a remnant of the obvious, noticeable slowdown/stop that numerous witnesses described seeing. For various reasons, this event had to be removed from the film. Throughout the current film the limousine seems to move at a steady speed. No stop or marked slowdown can be seen by viewing the film at normal speed, which of course is the speed at which the witnesses would have observed the limousine. Alvarez only detected the slowdown by making measurements based on a frame-by-frame analysis of the film. Until Alvarez discovered the split-second slowdown, nobody had noticed it. So surely this is not the slowdown event that the witnesses described.