Continuity mistake: Before the clown attack, its left leg keeps changing positions between shots.
Continuity mistake: The night after burying the bird, Robbie's hat is not tucked in, however a frame later it's totally tucked in.
Continuity mistake: When Carol Anne goes to the TV in the parents' bedroom the first time, the clock on the TV changes from 2:38, 2:58, 2:38 and then back to 2:53 with each camera angle change.
Answer: Ectoplasm.
Phixius