There is a pendulum clock in Prospero's castle although clocks of this kind were not invented until centuries after the medieval time frame of the film. Also of note: a close-up shows that the clock's pendulum has a blade-like bob at the end of the rod, similar to the blade at the end of the rod of the pendulum in Poe's story, "The Pit and the Pendulum" and the 1960 Roger Corman film inspired by that story.
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utilizing anything already available in the studio to dress his actors and sets, many of the men's hats are 300 years out of date with the time period that the story is supposed to have taken place in.
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.
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.
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.