Visible crew/equipment: When Dennis Weaver is in the cafe a shadow of a cameraman is cast across the table he is sitting at. The shot then changes to a wider angle to reveal that there isn't anyone near him who could have cast that shadow.
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: When the judge is interrogating Margaret, a very un-18th century filling can be seen in her teeth.
Visible crew/equipment: In the lower left-hand pane of glass in the kitchen cupboards, a crew member can be seen moving a rat (Ben) from the center table to where Willard left the box of rat poison. (01:30:42)