Continuity mistake: When Gwen and the police are at the house containing the buried bodies of the victims while Finn and the Grabber are fighting, the Grabber's van is not in the carport. However, when Finn exits the house, and he and Gwen embrace, it is. (01:28:03 - 01:34:27)
Continuity mistake: Spoiler Alert: When Finney digs the hole 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)
Suggested correction: I agree that the appearance of the hole changes, but I wouldn't say it went from "relatively round" to "almost perfectly square." (The appearance is probably due to the camera's angle). The hole started out as oval-like but progressed to a rectangular shape with curved corners. When Finney came back, the rectangular shape had squarer corners and straighter/neater edges. [The wording should be changed for accuracy, but I won't edit your wording.].
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)
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)
Suggested correction: It's a cable (which is also what The Paperboy calls it) not a rope. The window bars are maybe 8 feet from the ground, not 8 feet above Finney's head. It's quite possible that a thick cable could be pushed up through the rolled up carpet. It's also possible that there's a little mystical help taking place, maybe from the dead kid that was talking to Finney via an apparently broken phone.
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 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)
Character mistake: In the very beginning at the baseball game the runner is running to second base and the fielder doesn't even try to tag him out. Just runs right by him and gets the ball to the pitcher. (00:00:53)
Suggested correction: The center fielder missed the ball and the right fielder got it. The second baseman was at the border of the infield and outfield and the right fielder may not have noticed that the short stop moved to cover second base. By this time, the players might have been more concerned over the opposing team scoring a run because a player had advanced to third base. It was a "revealing" mistake or overall "stupidity" by the team as each player was playing "musical positions" more so than good baseball.
With regard to "revealing", the players may have been instructed to be active on the field and cover bases for each other as needed and this was the "mess" they produced.
Suggested correction: Baseball players make mental errors all the time, even major leaguers.
I've watched the scene a few times and can't agree or disagree with the mistake, but it certainly doesn't look like a mental error one would see in a game, even a little league game. It just looked like two kids were given directions what to do and the timing might have been off and the director on set didn't bother changing it. (I'm not disagreeing or agreeing with the correction either, just making a statement to help others decide).
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)
Suggested correction: What the viewer sees and what Finney sees would be different; his eyes probably see low level light better than the camera. There would be a low level light going into the corridor from the room Finney was in. This would possibly be enough for him to see the light cord. We don't see the light cord, so we don't know how easy it was for Finney to see it.
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.
Suggested correction: "Davy and Goliath" aired on Saturday or Sunday mornings, depending on the TV station. And, according to Wikipedia, it "premiered in syndication on 2/25/61 as a Saturday feature."