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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)

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Stupidity: Grabber made the basement room soundproof but failed to adequately cover the ground-level window so that nobody could see into the room. (00:35:15)

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Other mistake: After Finney got off the mattress, he was walking in the dark (at least a few steps). The screen went completely black, but Finney - who had not been in that basement before - was able to reach up and pull the cord to turn on the bathroom ceiling light. (00:29:50)

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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)

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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 telephone line was barely discernible against the dirty/marred wall but was obvious after Finney tried to use the phone. (00:28:50 - 00:30:21)

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