Factual error: Will's Range Rover has a rear New York license plate but not the required front plate. (00:08:25)
Continuity mistake: Lynn Jackson collects things from the desk and looks at George Pierce, who tells her "I'll talk to him", clutching his smartphone to his chest. Next shot and his arm is down his side. (00:18:10)
Character mistake: In the highlighted newspaper article entitled 'DeFeo Son Accused' about the killings, the first sentence of the article ends with two periods. Also, in the 3rd sentence, the word 'psychiatric' is spelled "psuchiatric" with a 'u' instead of a 'y'. (00:01:05)
Plot hole: Glenn's brother George and/or other relatives (Grandparents?) would question the validity of the document Glenn signed for Susan to get permanent legal guardianship of Christina. Also, a Director of Social Services (George) would request an investigation into Susan's background to ascertain if she is suitable to be Christina's legal guardian. A single (inept?) mother of one (or two), with a yearly income around $31K and a history of drinking alcohol to excess would raise red flags.
Continuity mistake: Alison is saying goodbye to her parents and she is wearing a long kimono. As she walks into her bedroom she is no longer wearing it, and then picks it up off her bed and puts it on. (01:07:05)
Deliberate mistake: The front passenger's side mirror [driver's side is on the right] is parallel to the screen/camera yet there is a perfectly clear image of the passenger's face looking into the mirror. (00:00:38)
Continuity mistake: During the Memorial Day toast, Katie's hand is on and off the glass between shots. The level of wine in the glass also changes inconsistently. (00:12:20)
Continuity mistake: Charlie was standing in the hole he dug and his head was below ground-level, but when Eliza was talking to him, Charlie's head was above-ground. Half of Charlie's back was above-ground when his mother [Toni Collette] was outside. Front views of Charlie standing in the hole still showed him against dirt (head below surface-level) even after he was just shown with his head above-ground. (00:51:10 - 00:51:46)
Character mistake: Set in Texas, the Sheriff threatens the wrongdoer with the gas chamber. Executions in Texas at that time were all by electrocution, namely by "Old Sparky," the electric chair. Texas has never used the gas chamber for executions.
Factual error: No matter how sophisticated the STEM chip is, it would not enable Grey (Logan Marshall-Green) to go from a completely horizontal position to a completely vertical position in one fluid motion, without bending his knees, and he does this several times in the film.
Revealing mistake: The bartender (Kateryna Korchynska) offers Ethan two lattes on the house. She hands them to him, and he walks out. No opacity from the liquid can be seen inside the plastic cups, and when he is moving away from the counter he leans forward quite a bit, showing the inside of the cups, squeaky clean. (00:08:00)