Revealing mistake: When Henry throws the dummy off the bridge, watch the rear window of the gray car; it explodes before being hit.
Visible crew/equipment: In the scene towards the end of the movie where the mother/aunt is holding onto Henry (Macaulay Culkin) and Mark (Elijah Wood), as they are dangling over the cliff, if you look closely at her holding onto Mark's hand, you can see a wire extending from under the sleeve of his coat to his aunt's.
Visible crew/equipment: When Henry pushes Susan off the cliff, look closely at her left side. You can see a wire attached to her hips when she falls.
Continuity mistake: When Henry attacks Connie for coming into his room and Mark steps in to stop him, they start fighting and Connie runs downstairs to tell on them. As the shot alternates back and forth, Henry's right hand jumps from Mark's ear to grabbing a handful of his hair and then back down again, but we don't see either boy move in a way that would explain the switch.
Revealing mistake: In the shot immediately following the multiple car collision, there are no people either in the cars or on the road.