Factual error: The teacher sends her pupils to the woods to collect chestnuts. Later we learn that the date when all this happened is May 30th. Chestnuts ripen in autumn, around September-October. (00:52:18 - 01:05:06)
Revealing mistake: When M3gan grabs the dog through the hole in the fence, the face of the person is not M3gan. (00:37:31)