Continuity mistake: During the whack-a-robot game, a big chunk of the broken robot lies on the counter, but it disappears a shot later.
Continuity mistake: When Josh's mom is about to cut the Butcher/Millie out of the chair, pay attention to her right hand, which is holding the knife. Right before the Butcher/Millie springs up, as she's talking to her son, her arm goes from down by her side with the knife pointed down, to up by her chest instantly between cuts.
Continuity mistake: When the boy is about to eat his corn bread, the knife on the left keeps changing positions between shots.
Character mistake: There are signs out front as the Spenglers arrive at the dirt farm. One of the signs is a call back to a scene in Ghostbusters 1984. In the scene, Ray quotes the book of Revelation: "And I looked, and he opened the sixth seal, and lo, there was a great earthquake. And the sun turned black as sack cloth, and the moon turned blood." When Finn Wolfhard reads it he misquotes the line saying "The sun became as black as cloth."