Joanna refers to herself as a 'Kindergarten teacher', though she is British and that term is not used in Britain. She would have referred to herself as a 'nursery school teacher' or 'primary school teacher'.
When Dr. Holden meets Joanna Harrington at the funeral, she says she needs to talk to him. He suggests that they meet at his hotel that night. She agrees and leaves. At no point did Holden tell her which hotel or room he was staying in. The next scene, she arrives at his hotel room. There is no evidence that the newspapers reported where he was staying, so how did she find which hotel without his help? London does have more than one hotel.
In the opening shot, when Professor Harrington asks to see Karswell, he demands "I must see Dr. Karswell at once". When Bates, the butler replies, he refers to Karswell as "Mister", and nowhere else in the movie is Karswell addressed as "doctor". One would assume that Harrington of all people, having headed an investigation into Karswell and his cult, would know his title.
At the beginning, when the doomed Professor Harrington is driving to Dr. Karswell's place, the steering wheel is on the left, and the POV is from the shotgun seat, looking at the right side of his face. On his way back to his home, it's a mirror image of the original (flipped shot).
Runic Parchment: When Holden first discovers the parchment which Karswell has passed to him, it has 2 rows of runic symbols. Moments later, when the parchment is trapped against the fire guard, it only has one row. Again, there is only one row on the parchment when Holden visits Stonehenge.
When Harrington sees the demon and begins to back out of his garage, a sign on the power-line pole clearly states, "Danger- Danger, live wires", yet when he backs into the pole a moment later the sign, which should be facing the camera, is now missing.
At Stonehenge, Holden pulls out his pocketbook, inside of which he has tucked the runic parchment. He unfolds the parchment and holds it up to compare the writing with that seen on the stone wall in front of him. The camera angle from behind shows him holding the edge of the parchment, as well as his pocketbook, in his left hand. Then the camera angle changes to a frontal view, but now his left hand is clutching only the edge of the runic parchment, with no pocketbook visible at all.
On the railway car, Karswell has his arms crossed across his chest. On several shots he has his left hand tucked inside and a few shots of his left hand on his right elbow. This is quite noticeable in the shots when Holden is trying to hand Karswell a piece of paper just after Joanna comes out of her trance.
When Karswell is chasing after the parchment, passing through several train cars in the process, the sound suggests that the train is still moving and at high speed. In the next cut Karswell is jumping out of the train that has suddenly come to a full stop at the station. The transition between fast moving train and stopped train is too abrupt.
When Karswell is chasing the parchment along the railway line, the parchment settles on the left, as he sees it, of the rail. After it has ignited, it is on his right, and he crumbles the ashes with his right hand. When he looks up, the station is behind him, which means the ashes would be on his left once more.
At the end when Dr. John Holden passes the parchment to Dr. Julian Karswell and it flies away, a thin wire is visible that special effects used to guide the parchment along the tracks as Karswell chases it.
When Holden visits Stonehenge the shot pans from the car toward the Stones. The car's door is left open, wavering in the breeze. Yet this is the passenger door, and as Holden is carrying nothing save the runic parchment, and he is alone, there is no reason for this door to be open.