Plot hole: The police Inspector recounts the murders, comparing them to the ten Biblical plagues of Egypt, and he states that in keeping with this theme Dr. Kitaj was killed by rats. First, Dr Kitaj dies in a plane crash. Okay, he was attacked by rats in flight but nobody would know that - his plane crashes in a huge, violent fireball and nobody would find any rat remains after that. Second, there was no plague of rats in ancient Egypt! (Go ahead, look it up). Even if the police had made the connection between the mysterious deaths and the ancient plagues they would not make the connection here.
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.
Question: One of the doctors Phibes blames for his wife's death during an operation is Dr. Hargreaves, whose head is crushed inside a frog mask during a costume ball. But earlier, while introducing himself to Phibes, Dr. Hargreaves says he is a psychiatrist ("headshrinker"). Why would a psychiatrist be present during an operation to save someone's life?