When David is hitting all the foul balls, one hits and breaks a pot of flowers, but in the next shot the flower pot is back where it was.
When the Space Shuttle model takes off, it flies in a high arc away from the launch site like the real one, but when the boosters fall back down they fall right back down where they launched from.
When Fingers hands the baseball bat to Mac in the tunnel, he puts it in the hole handle end first. In the view from inside the tunnel the bat comes in fat end first.
When the boys arrive to see Smalls on the sandlot for the first time, and they all charge toward him on their bikes, Fingers is seen getting off his seat just as they charge. However, the very next shot, he is back on his seat.
When The Great Fear's chain is welded to a Battleship anchor, the anchor is huge, larger than the person welding it. When you see the anchor in all the shots afterwards, where the kids try to get the model, the anchor is tiny about 20% the size of the one welded on in the beginning.
When David grabs his red rocket coat he tucks it under his shirt. When he is going on his bike it is still under his shirt but when he starts riding away it is in his back pocket
When David is running out of the "fun house", his right sock is pulled up to his knee. Seconds later, the scar is showing and he is pulling up his sock from his ankle.
Obvious stunt double when the dog is chasing David into the junk yard.
One of the missiles that Johnny launches lands on the roof of there club and knocks it in. But later it is as before.
As the model Space Shuttle flies over the city, it goes over a Foot Locker. The movie takes place in 1972, Foot Locker did not exist until 1974.
At the end of the movie, the narrator explains that Hayley grew up to be a supermodel and a pitcher on the Olympic softball team. Women's softball did not become an Olympic sport until 1996. Hayley would have been around 35 years old in 1996, a very unlikely age for an Olympic softball player.
The model of the space shuttle includes a rust colored external fuel tank. In the early flights of the space shuttle the tank was painted white. Most likely if any models existed during development of the shuttle they would have had the tank painted white.
When the kids were fist upset that the girls were at the sandlot, one of the boys asked another boy "Are you on crack?". This movie is set in 1972 but crack was unknown to most people until 1981.
When The Great Fear throws Small's toy over the fence everybody is screaming, When it cuts to a different position they are still screaming but their mouths are closed.
When Mr. Mertle's wall gets knocked over you can see two black wires that are obviously pulling it over.
The bet made by the kids near the start of the movie should've been won by the boys. Hayley's exact wording is "I'll bet I can strike him out with three pitches". The first pitch is a strike, the second is a foul ball, then there are so many consecutive foul balls that in the end they get tired and call it a draw. So clearly the boys won, because she didn't strike the batter out with three pitches.
At the first encounter over the sandlot field, when Mack states "Are you deaf?" to Samantha, he turns and apologizes to Fingers. Since Fingers is deaf, and Mack was not facing him, there is no way Fingers could know what he said.