I read that book cover to cover when it came out (still have my copy somewhere), and I must say that the author makes a pretty compelling case.

The main theory is that someone (maybe Oswald, maybe someone else) was shooting from the book depository, but they only got off two shots. The 3rd shot was from the rifle of a Secret Service agent in the convertible directly behind Kennedy, which went off accidentally as he turned to return fire to the book depository.

The author shows how the bullet that blew off the top of Kennedy’s head was a soft metal jacket like the Secret Service used, while the first two shots were hard metal jackets, which was what was found in the depository. (He goes into great detail on the ballistics.) Because of the shape and acoustics of Dealy Plaza, a gunshot coming from the car behind Kennedy could sound like it came from the grassy knoll. Lastly, it would explain why people in the plaza claimed they smelled gunpowder, which wouldn’t be possible if the only shots came from the book depository.

The author posits that the two shots that got Kennedy from the book depository might have killed him anyway, but the 3rd and fatal headshot was an accident from the Secret Service agent who was sitting on the back of the convertible in front of Kennedy with his feet in the back seat. He stumbled as he tried to stand in the seat to turn and return fire to whoever was shooting from the book depository behind him, and the rifle accidentally discharged, thus the title of the book Mortal Error.