Chalk marks are visible on the wall during Rod Serling's opening. They are drawn there later on, showing that the intro was shot later.
While in the other dimension, the little girl can be seen holding the dog's collar. In a subsequent shot, it is seen that her nightgown is tied to the dog's collar. At the end, she is seen holding the dog's collar again.
When Bill draws chalk marks on the left-hand side of the 4th dimension entryway to mark it, they have disappeared when the camera shows the wall again after his close-up.
When the camera angle changes just before Bill puts his hand through the wall for the second time, Ruth's hand instantly moves from one side of her face to the other.
As the camera pans up on Rod Serling for the opening narration, he can be seen glancing to his left for a second to receive his cue to start talking.