Despite Prospero warning the guests not to wear red to the masque, several people are wearing red: capes, hats etc.
Clear discontinuity between shots when Read Death asks Prospero why would he be afraid to die.
A clear discontinuity between shots just before Prospero takes of the mask from the Man in Red's face.
Francesca (Jane Asher) uses a candle at the beginning of her walk but not as the walk continues.
At the beginning of the movie, when Man in Red calls grandmother to come closer, he is looking at her, but in the next shot isn't.
The use of an inverted cross as an anti-Christian or Satanic symbol is largely a creation of 20th century movies. The cross is known as the Cross of Saint Peter, who was one of the Twelve Apostles. When he was to be crucified Peter asked to be done so upside down as he was unworthy to die in the same way Jesus did. During the Middle Ages and Poe's time it was a holy symbol no Satan worshiper would use.
The gorilla suit used in the film is obviously something made in the 1960s, and is a modern prop and wouldn't have existed in the time the film takes place.
When guard is thrown to the wall, the wall shakes, revealing it is fake.
When Francesca (Jane Asher) awakens from her nap, she picks up a green candle from the bedside table and uses it to light her way in spite of the fact that the room is not dark. Despite the heavy wind blowing in from the open balcony, the flame of the candle does not blow out.
When Gino is climbing the castle wall, the whole wall shakes, revealing it is fake.
Towards the end of the movie, just before Prospero sees the man in red, a blonde woman dances from left to right. The frame then jumps and the blonde woman is once again on the left side without having returned.
When the man is shot with the arrow, Vincent Price is clearly shooting the crossbow off screen. In the next cut, see
that the man fall from arrow shot, not only is there hardly any blood, but he was clearly just holding it there, ready to fall down for the take. Very obvious in High Definition.
True to Corman's penchant for utilizing anything already available in the studio to dress his actors and sets, many of the men's hats are 300 years out of date with the time period that the story is supposed to have taken place in.
The accent used throughout the films are clearly American accents, and many of them wouldn't have existed at the time, clearly they just hired some English actors and Americans to do the film, and had no concern for historical accuracy.
When Francesca, Gino and Ludovico escape the dungeon, they encounter Prospero waiting for them outside. Prospero immediately states that Gino and Ludovico will have to answer for killing three of his guards while escaping the dungeon below, but as he was outside, there is no way for Prospero to know if the guards were killed. The guards could have been merely injured and not killed, or bribed, or they could have let them pass without a fight.
Prospero gives "Belial" as an alternative name for Satan. In virtually all sources this is the name of a demon, not Satan himself.
Prince Presporo clearly speaks with a very pronounced American accent. Not only is this impossible, but he should be speaking with a natural European accent, but the accent that he uses in the film, is also a modern accent and indicates that Vincent Price was indeed born in the United States.
Many of the actors clearly speak with American accents. Many of them don't even attempt to sale some kind of European accent, but just speak naturally. Vincent Price also speaks with his regular American accent and it can be heard clearly.