Teddy's left ear (which his father almost burned off) in the tree house scene is very clearly burned. Throughout the movie, however, it varies in how it looks, in the train tracks scene it looks perfectly normal.
When the boys are crossing the bridge, and Gordie is trying to help Vern to his feet there is a very brief cutaway showing that the oncoming train is only the engine and one car. The previous shot, and the later one once it passes, all show it to have several cars.
In the clubhouse at the beginning of the movie, Chris has a pack of cigarettes tucked into his sleeve. It disappears and reappears throughout the scene.
Teddy's hair changes in every shot after being dunked in the swamp. The strap to his bag disappears and reappears as well.
When we're first introduced to Teddy Duchamp in the tree house, we see his badly burnt ear but also on the same side of his face he has a big mole at the side of his nose. When they set out on their adventure, it's disappeared and never seen again in the movie.
Near the beginning of the movie, Vern is described as burying then losing a quart jar of pennies "at the beginning of the school year," and searching for it for the last nine months. The story takes place Labor Day weekend, so that description would indicate Vern did not start looking for the missing trove until early December. It doesn't mean the story was at one time set at Memorial Day.
After cooking supper around the campfire, there is a shot of a full moon. But on this night, Friday September 4, 1959, only 3% of the moon would have been visible, a "waxing crescent" moon.
When the boys are camped out in the woods thru the hot summer night, there isn't one scene of them fanning away pesky mosquitos, ants or other insects from the summer heat and wilderness..
The extras wearing the BOA hats in the pie-eating sequence are not always sitting in the same order. This may be deliberate, to depict the absurdity of Gordie's imagination.
Gordie is not looking for his hat in Denny's room, he only reminisces about it there. It wasn't in that room, so it's not an error for him to be wearing it later without taking it from the room.
Ben E. King's "Stand By Me", which the film derives its title from and uses as its theme song, was not recorded until 1961. However, it is non-diegetic (not heard by the characters), and it is also first heard when Gordie is an adult in the film's opening scene, which would be well past 1961.
In the "barf-o-rama" scene, all the actors vomit up blueberries, just like Lard-Ass and the other contestants in the pie-eat. Nobody in the audience (at least from what is shown) ate any blueberries. However, this is a silly story which Gordie is imagining, and he hasn't got everything right in his mind.
When Gordie shoots the garbage cans and runs off, he's still carrying the gun. After a scene cut, they're running when they get around the corner and he's not holding the gun anymore. It's safe to presume they stopped, put the gun away, then again took off running to vacate the area quickly.
When Gordie looks at the newspaper article at the beginning, the first paragraph of the article is about the stabbing, but the second column is clearly from another story.
In the "barforama" scene, the jets of vomit do not quite seem to come from out of the mouths of the actors.
In the train scene, River Phoenix' voice has suddenly changed and he looks older. This scene was obviously shot last, and he has started going through puberty. But, in terms of movie time, he has gone through puberty in two days.
When Lardass Hogan swallows the egg, it instead falls down the side of his face and can be heard hitting the ground.
In the car race, a log truck is rapidly approaching Ace in the oncoming lane. Four shots of the log truck are shown as it gets closer. The first three shots show the road with no visible roads or driveways connecting. Then on the forth shot, a road/driveway (that was never there) suddenly appears, allowing the log truck to veer off the road at the last second.
The whole plot of the film was that four young friends, along with a gang of older teens, set out for the woods near their local town, to find the body of a teen who was run over by a train. It's hard to believe that the local police wouldn't have done a search and recovery and discovered the teen long before anyone else had. After all, the child was reported missing and news of his disappearance and possible death by train was all over the local radio..
When Gordie and Chris are discussing the stolen milk money, Chris uses the term "douchebag" in a connotation which did not originate until 1967.
The Topps baseball card in Denny's room is much more recent than 1959.
1980s cars visible in the background of the junkyard.
When Gordie is in the delicatessen there is a bottle of Soft
Soap in the background that was not available in 1959.
When Gordie visits the delicatessen for ham, the grocer pulls waxed paper from a box with a 1980s Crown Zellerbach logo on it.
When Vern is first coming into the tree house and is coming through the trap door he is clearly heard saying "You guys are never gonna believe this...", but his mouth is saying the previous lines "Oh, man, oh, man".
In the first train dodge scene, after Teddy and Chris make up, as Verne walks off back onto the train track you can see his bright white wireless mike drop down his left leg and flop near his ankle.
During the "barf-o-rama" scene, the two twin brothers barf on each other. As the second brother barfs, the other brother turns his head and you can see the white tubing of the barf mechanism on or around his left ear.
After Ace takes Gordie's cap, the reflection of a crew member can clearly be seen waving their arm in one of the store windows behind Chris and Gordie.
The camera tripod can be seen in Teddy's glasses when Teddy and Chris are arguing on the railroad tracks.
In the tree house scene, right after Chris says to Teddy, "come on man deal", Teddy blows smoke from his cigarette while laughing at the same time. As he's doing this, we can clearly see a film camera right next to Chris' head in the reflection of Teddy's eyeglasses.
Like many of Stephen King's stories, the original story takes place in Maine. Although relocated to Oregon, the movie has several holdovers from the original location, such as the reference to picking wild blueberries, which do not grow in Oregon. Also, the junk man teases Teddy by telling him is father will be "sent to Togus". Togus, Maine has a large veteran's hospital, which would be the logical place (in Maine), for Teddy's dad to get treatment.
Like the reference to "Togus", the Royal River is in Maine, not Oregon. Stephen King refers to the Royal in other stories, as he sets many of his stories in Maine, where he grew up.
At one point in the movie, Gordie says that Billy and Charlie were playing mailbox baseball with Ace and Eyeball. There's no way that Gordie could know that as he was with his friends the whole time.
When Gordie goes into the store and the clerk talks to him, you can cleary see the boom mic bobbing into view and then being taken away.
When Ace is playing pool with Billy, the boom mic bobs into view, near the light.
Immediately before the boys start to cross the bridge.
When they're in the clubhouse at the beginning, Gordie looks straight at the camera while singing "I Ran All The Way Home".
Chris says his pistol is a .45 but when he first pulls it out of his pack the barrel is clearly much smaller revealing it to probably be a prop gun.
Chris suggests using the Route 136 bridge to cross the river, which Teddy opposes, claiming that that bridge is 5 miles down the river from the trestle. In real life, there is no Route 136 running anywhere in Oregon.
Gordie describes Lard-Ass as being "really fat; weighing close to 180." 180 pounds really isn't that heavy. When Lard-Ass is seen, it is obvious he weighs well over 300 pounds.
If you look closely to the right of Ray Brower's neck , you can see he still has a pulse.