Plot hole: Dr. Phibes kills Dr. Hargreaves at a costume ball by crushing his head inside a frog mask. But one of the plot's main points is that Dr. Phibes is presumed dead following the car crash in Switzerland many years ago. So whose house is the costume ball at? If Phibes is presumed dead by all his friends and colleagues, why would they show up to a party at his house? Or did Phibes simply crash someone else's party, knowing (or hoping) that Dr. Hargreaves would be there?

Revealing: When the bats invade Dr. Dunwoody's bedroom, you can see the strings on one of them.

Continuity: Dr. Phibes drains Dr. Longstreet's blood out of him while Vulnavia performs a violin solo. Later, when Dr. Longstreet's corpse is discovered and the police question his housekeeper Mrs. Frawley, she says that she heard a violin playing in the street "about half past 2 in the morning". But earlier, when Vulnavia was sitting outside playing her violin, it was broad daylight.

Factual error: 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 Egypt. Frogs, lice, boils, yes, but no rats.