Plot hole: If the Lady of the Lake walks every night to her room and kills those in her path, why didn't Flora and Miles' parents die in their room by the hands of the Lady in the Lake when they first moved in?
Continuity mistake: Kurt is still recovering from the beating he took in the previous episode with bad cuts on his forehead, checks and temple area. They are red and impossible to miss while he is in the police station. The cuts have miraculously disappeared in the next scene when he goes to visit Mona.
Factual error: The show, which is set in California in 1947, mentions the gas chamber, lethal injection and the electric chair as methods of execution in the state. Two murderers are sentenced to death by lethal injection and one is depicted being instead executed in the electric chair, which the governor states has been brought out of retirement for the occasion. In fact, California adopted the gas chamber as its sole method of execution in 1937 (having previously used hanging). Lethal injection was not used in the United States at all until 1982 and not adopted in California until 1993 (becoming the prime method in 1996). California has never used the electric chair.
Continuity mistake: Throughout the series, the Barbers pull their cars straight up their driveway. But the cars are nearly always shown coming back down the driveway driving forward.
Revealing mistake: There are bullets placed on the counter, the one casing has a hole in it from being drilled to show it's a dummy round and there's no gunpowder in it. (00:14:45 - 00:15:20)
Factual error: A man enters a bank and begins firing his Glock 9mm pistol. A few of the men he shoots, jump up in the air after the rounds impact their bodies. Bullets hitting a body will never make it jump, or fly in the air. (00:00:01 - 00:01:46)
Factual error: A British soldier in 1940 says he was educated at the University of Warwick. The university wasn't founded until 1965.
Episode #1.7 - S1-E7
Other mistake: Surveillance cameras are shown on monitors when the mass panic kicks in even though any chip-dependent electronics shouldn't work in the game world (previously stated by Chotu). (00:08:20)
Factual error: In Massachusetts, Atticus, Leti and George are attacked by the local sheriff and his deputies, who are patrolling the county roads. In Massachusetts, sheriffs run the county jails, but they do not (and did not) handle routine policing, which is the responsibility only of municipal police departments.
Continuity mistake: When Eden confronts Kinsey by the lockers, as the angle changes so does Kinsey's arm. In some shots, she has her arms crossed and the locker door is closed, in some shots she has one arm up, holding the (open) locker door.