The film is set in 1925. The automobiles, airplane, and film projector seem to be from the 1920s, but the house interiors, including the lights around Dr. Phibes' organ, and clothing appear to be early 1970s "mod" style. At the end Dr. Phibes plays "Somewhere Over The Rainbow" which was written in 1935.
The organ Dr. Phibes uses is clearly meant to be a Wurlitzer style theatre pipe organ, complete with elevating mechanism. The sounds we hear at the beginning are clearly from a baroque style church organ, which has a different action and pipe rank arrangement. Later in the film the sound clearly comes from a Hammond tonewheel organ with rotary speaker cabinet (Leslie) on slow speed (chorale). The latter was introduced in 1941.
During the operation scene, Vulnavia is seen cutting a hole in the painted backdrop through the face of a seated man. Inspector Trout and Tom are subsequently seen putting their heads through two slashes in the backdrop, one of which is located above this hole. But after Vulnavia's death, as Trout orders his police constables to search the house, the holes for their heads have disappeared.
The Ten Plagues of Egypt are wrongly depicted, both in order of appearance and what they actually were. According to the book of Exodus, the Ten Plagues were: 1. Water to blood, 2. Frogs, 3. Gnats, 4. Flies, 5. Pestilence (upon the beasts of Egypt), 6. Boils, 7. Hail, 8. Locusts, 9. Darkness, and 10. Death of the Firstborn (Exodus 7:14 - 12:36). The correct names are written in Hebrew but the rabbi reads "pestilence" as "bats" and "gnats" as "rats". The first and second plagues on the scroll are moved down to be the fifth and sixth plagues. Finally, the rabbi incorrectly reads the plagues from left to right instead of right to left as is normal for Hebrew thus changing the order of the plagues even more so.
We learn that the doctor's face is disfigured so badly that he cannot open his mouth anymore and instead he has a gullet opening at his neck. However, he could not eat any solid food (since he could not chew it), but he does so in the movie.
Virginia North smiles broadly as she dances with Vincent Price. Her face is concealed most of the time, but if you look closely, you can see her face for a moment, and she is clearly trying not to laugh at Vincent Price's deliberately camp performance.
The goof items below may give away important plot points.
In the closing sequence, when the surgeon's son is chained to the operating table, Dr. Phibes says several times that he will have 6 minutes before the acid falls. The continuous, uninterrupted sequence runs 5 1/2 minutes from the time Dr. Phibes starts the flow of the acid to the moment it falls in Vulnavia's face.
During the murder of the nurse, when Phibes puts the outline of her body on the floor and drills a hole through the head, he looks through the hole he drilled but the view he has of her has her body lying in the opposite direction from the outline he laid on the floor.