The scars on Harold Finley's (Donald Pleasence's) head change from scene to scene and sometimes aren't there at all.
Harold's stormy id is special effect footage added in post. The actors react to it, never able to actually see it. The appearance of the id, unfortunately, often becomes mistimed, with actors "seeing" it when it isn't there or not reacting strongly enough when when it's later added in. (Actors are especially under-expressive during the climatic attack.)
One of two workmen signals to a third man who is never there.
At 31:31, the lights mounted on the power columns are nothing more than 3 different size mid-century ceiling light globes stacked on black painted cardboard rolls. The seams on the cardboard rolls can be easily seen. I didn't noticed it when I first watched in October 1963, I was 5, sitting in front of a poor resolution B&W TV. I can see it now with my 4K 80"....
In the Finley living room, Harold and Vera react to seeing Harold's stormy id before his id appears.
When Finley levitates the rock, the wires holding up the prop are visible.
When Harold's monstrous id visits the Radcliffe bedroom, Emily is asleep, visually dead to the world in seconds as the Dean is vaporized. She later reports that she was pulled out of bed and thrown against the door. What's said and what's shown differ.
One of the scientists at the space center wrongly describes titanium as a fissionable element.
Vera Finley falls off a ladder and says she was pushed by an invisible force. Not long after she says she was pulled off the ladder.