When the doctor performs an operation on Diffring's thyroid, he makes the incision somewhere below the waist. The thyroid is actually in the neck.
The prostitute says to George to watch out for the 'dust bins' in the dark alley. The term is common to England but not France. However, since the characters are presumably speaking French anyway, and the English dialogue is just a translation for the benefit of the audience, using a British term in a British movie is not a goof.
Christopher Lee's hairline raises and lowers from scene to scene.
When Margo throws the oil lantern at Bonnet, the flammable liquid which ignites is visible on Bonnet's jacket before the lantern hits.
Anton Diffring's make-up at the end has obvious eye holes revealing the actor's own face, and thus is clearly a mask rather than the aging of the character's own face.