Continuity mistake: When the boys are in the treehouse at the beginning, the coca cola bottle on the treehouse shelf constantly moves places between shots. The two sides of the bottle saying "coke" and "coca-cola" keep switching positions between shots also. (00:02:55 - 00:04:55)
Audio problem: At the beginning when Vern comes to the tree house to share his exciting news, when he is climbing up to get inside his lips do not match what he is saying. (00:03:40)
Other mistake: In the treehouse scene, Teddy tells Vern that nobody will care if they didn't go to the drag race. When Teddy says this you can see Chris mouthing his lines. (00:07:45)
Factual error: When the boys start their hike at the railroad yard, there is a modern transformer on a pole in the background. In 1959, they were smaller and black. (00:15:00)
Visible crew/equipment: In the scene just after the train dodge attempt by Teddy, you can see something white hanging out of Vern's pants as he walks onto the tracks. It is probably some part of equipment for his mike. (00:17:50)
Continuity mistake: All four guys are in the junk-yard and they are all about to toss their coins to see who goes to get the food. When Teddy throws his coin in the air he is wearing no dog tag necklace but when he catches the coin he is suddenly wearing the dog tag necklace. (00:22:15)
Continuity mistake: When the boys are at the junk yard and they nominate Gordy to get the food, when Gordy jumps up and starts joking with the boys, Vern and Chris start to fight. Then the shot changes and suddenly Teddy, who was off to the side, is in the middle of the pile. (00:23:35)
Factual error: There are aluminium cans in the general store. The first all-aluminium cans of this style weren't used in 1959, when the film is set. (00:24:05)
Factual error: When Gordy goes to get supplies at the store, as he is standing there the clerk is talking about Denny, and in the background on the shelf there is a bottle of Soft Scrub. It's supposed to be the 1960s, but Soft Scrub was not around until the early 80s. (00:25:00)
Continuity mistake: When Gordy returns to the junkyard after getting the food he sees his friends running away. The junkman yells at him and Gordy starts running away towards the fence. In the background he passes the same junk vehicles multiple times. (00:26:15)
Continuity mistake: The amount of blueberries and sauce on Lardass Davey Hogan, before he vomits, changes from shot to shot. (00:46:10)
Revealing mistake: When Lardass pukes on the guy to the left of him, he moves his head prematurely, and you can see that the vomit is coming out of a jet (hose) positioned behind his head. (00:46:30)
Continuity mistake: When the boys get dunked in the swamp, Teddy takes his glasses off, but then while they're thrashing around in the water, both hands are clearly empty. Then, when they get out, he has his glasses in his hand again. (01:02:05)
Continuity mistake: When Gordie, Chris, Vern and Teddy fall into the water, Teddy and Vern go completely under, but Gordie and Chris are able to keep their heads above the water. In the next shot, all four of them emerge from under the water. (01:04:15)
Continuity mistake: When the boys are dunking each other in the swamp Gordie starts to wade away, when he passes the camera his hair is dry but when he reaches the bank it's wet again. (01:04:56)
Factual error: When they find the dead body, the body looks like it has been dead for only a short while, his face even had a pinkish color to it still. Factually, the body would be a lot more decomposed after a few days, bluish or white in color from the blood pooling at the bottom of the body, since the body was on its back. (01:08:30)
Revealing mistake: When Gordie cries after finding the body there are no tears in his eyes and his face is completely dry. (01:10:45)
Factual error: After Wil Wheaton fires a warning shot, he points the pistol at Kiefer Sutherland and pulls the hammer back to threaten him. However, this is a semi-automatic pistol, and the hammer would already be cocked after the shot he fired. (01:13:40)
Other mistake: Teddy's hair changes in every shot after being dunked in the swamp. The strap to his bag disappears and reappears as well.
Continuity mistake: In the beginning he mentions Teddy's left ear being burnt by his father on a stove. Notice how his ear goes from looking severely burnt, to not burnt at all, through the movie. Especially the water dunking scene.
Answer: A cello.