Revealing mistake: When Finney is looking for the bike lock's combination written on the wall, the distance between the mattress and that wall (several feet) can be seen. However, the next time Finney wakes up on the mattress, the mattress is against or almost touching the corner, but the mattress was not that close to the adjacent wall. Furthermore, when the Black Phone rings, the mattress is the same distance from the phone as it originally was (which would be several feet away from the wall). (01:04:59 - 01:10:53)
Factual error: At the breakfast table the father is reading the Rocky Mountain News newspaper, which is quarter folded (like most papers) but the RMN was always a bi-fold paper, like a large magazine, folded down the center. The RMN went out of business a few years ago, but they always used this format.
Revealing mistake: The angles of the adjacent walls are not always accurate given the relative position of the mattress on the floor. The mattress is perpendicular to the wall where the Black Phone is attached and at least a few feet away from the adjacent wall. However, when there's a side view of Finney with a front view of the Grabber (about to get up), the wall behind them should be parallel to the mattress but is at an angle. [The position of the mattress in relation to the walls can be seen later.]. (00:29:10 - 00:30:28)
Continuity mistake: The length of the Black Phone's cord changes. The cord that could be seen between the Grabber and Finney was relatively short and had a wide loop at the bottom. When Finney came back from the bathroom, the cord was hanging down about twice as far and the loop was not as wide, making most of the cord look linear. Also, the cut cord was barely discernible against the dirty/marred wall but was obvious after Finney tried to use the phone. (00:28:50 - 00:30:21)
Plot hole: Spoiler Alert: When Finney gets the rope that was hidden in the wall, he needs to thread it through the window bars about 8 feet above his head. After trying to lasso the rope through the bars, he grabs a rolled-up carpet and threads the rope from the bottom of the carpet through the bars and it wraps itself around the bars. There is no way a rope (or even a bendable cable) would be able to thread itself 8 feet through a carpet. After he grabs it, you can tell it's a loose rope, not a stiff wire. (00:51:15)
Continuity mistake: Spoiler Alert: When Finney digs the whole through the wall with the toilet tank cover, he creates a relatively round hole to the back of the refrigerator. After climbing into the hole and then back out, the hole is now almost perfectly square. (01:16:38 - 01:17:28)
Factual error: On Saturday morning Gwen and Finney were watching cartoons. This was the day after she spent the night at her friends house which was a Friday night. The cartoon that was on the TV was Davey and Goliath. This cartoon only aired on Sunday mornings.