Factual error: When Harry is confronting the bank robber and the robber says he has to know if Harry had any shots left, Harry cocks the hammer which rotates the cylinder. If you watch closely when he pulls the trigger the cylinder rotates again. This can happen only when the trigger is pulled in double action mode (hammer down). (00:14:00)
Factual error: In keeping with the theme of the Ten Plagues of Egypt, Doctor Phibes has one of his victims eaten alive by locusts. Not likely. Locusts are strictly herbivorous, and like all herbivores locusts won't touch anything that moves. They would not be attracted by the slimy goo poured over their 'victim', because they zero in on plant pheromones and nothing else.
Factual error: There are numerous references in the film about the type of jet, a Phantom, that crashes 60 miles outside of Piedmont, with a consultant at the crash site even saying "There's no rubber at all used in a Phantom F-4, General." The crashed tail-section of the plane at the crash site is and obviously not from a Phantom jet, but from a F-84 Thunderjet, a much smaller and older jet than a Phantom.