Continuity mistake: In between shots of Lardass as Gordie says "By the time he was on his fifth pie, Lardass pretended that he wasn't eating pies.", the crust on Lardass' forehead disappears.
Revealing mistake: Not to say Lardass was thin, but it is very clear that his shirt is stuffed up with some kind of padding to make him appear very obese, when he is announced for the pie eating contest.
Continuity mistake: At the end of the first shot of Lardass drinking castor oil, the bottle is nearly full, but at the start of the next shot, the bottle is nearly empty. Because of the people talking in the background, they couldn't have cut forward to the part where he is finishing.
Revealing mistake: In the car race scene with Ace and his gang you can see that it is not Ace driving when the car runs over the wood in the road.
Continuity mistake: During the mailbox baseball scene, Ace is shown with a cigarette in his mouth. Then after he strikes out and says "ah shit, I'm out", the cigarette is gone.
Continuity mistake: In the leech scene when the boys get out of the water, Chris takes off his shirt and there are no leeches on his back. But when Chris says "Gordie there are some on your back man. Are there any on mine." he has about 4 or 5 leeches on his back.
Continuity mistake: About half way through their journey the boys clothes are very dirty, then they become spotless.
Continuity mistake: When the four boys first come to the railroad, Vern shows them his comb. He is holding it upwards, then lowers it a little. Then when Gordie says "Lotta thinking Vern, thanks," his arm is straight down next to his side.
Visible crew/equipment: In a bird's eye view when the boys cross the bridge, there is a camera visible at the end of the bridge.
Revealing mistake: During the pie eating scene when the projectile stream of blueberries starts, the alignment is off for most of them, as it appears to come out of their noses and chins as well. The pie eating contestant next to the radio personality has their stream fall in an arc until it looks like it's coming from beneath their chin directly downward.
Continuity mistake: After Teddy tries to dodge the train, the boys head back in the opposite direction they were walking. Not a question of camera angles. They're walking towards the oncoming train. After the train passes, they resume walking in the same direction it passed.
Factual error: There's a scene with a house in the background that has the electric wires from the power pole going to a connection above the roof. In 1959, the connection would have been made under the eaves or on the outer wall.
Continuity mistake: The author in the beginning is supposed to be Gordy grown up, but the grown up Gordy has blue eyes and light hair color and Gordy as a kid had brown hair and brown eyes.
Continuity mistake: At the story told by Gordie about the kid eating pies, at the last pie his face cleans up a little bit between shots.
Continuity mistake: When the boys are continue their journey the second day they are by each other eating blueberries but when it cuts Gordy and Chris are in front of Teddy and Vern while the others are both holding a string.
Revealing mistake: When Ace runs the truck driver off the road, it's obvious that Ace Billy and Charlie are stunt doubles, because of the hair and age.
Continuity mistake: In the beginning scene, the newspaper clipping under the headline shows the first sentence to be about Chris' death from a stabbing, but the next line starts mid sentence about a completely different subject and the second column under the headline talks about taxes, mentioning nothing about the death.
Continuity mistake: All throughout the movie, the angle of the sun and shadows change within scenes. Especially in the scene when the boys are near the train bridge talking. As the camera shots switch, the shadows change angles.
Continuity mistake: Near the start of the movie, in the tree house scene, Chris has a cigarette packet rolled up in the left sleeve of his T-shirt, the cigarette packet is seen to disappear and reappear several times during this scene.
Continuity mistake: When the boys are roasting marshmallows, Verns falls off the stick in two pieces. When he picks it back up with the stick, it's whole again.
Answer: A cello.