Revealing mistake: When the boys' rival team challenges them, all the boys throw their gloves down, except Yeah-Yeah. When they all go to pick them back up, Yeah-Yeah has to pretend to pick it up.
Continuity mistake: When the treehouse is about to explode, we see Beltram say he's getting out of there and go down the ladder. Then after some scenes we see him running out of the tree house when he was supposedly already down on the ground.
Continuity mistake: When the fence is about to fall on the dog, it changes camera angles. In each camera angle the fence falls in different directions. Plus when it starts falling, the dog turns around facing the fence, but when it falls on him, he is facing the other way.
Continuity mistake: It is established that "The Sandlot" is a relatively vacant lot, mostly dirt, with grass mostly in the infield areas. But numerous times in the scenes involving trying to retrieve the baseball, the background shows a manicured yard with a yellow house just off to the side. It's not in the distance, it's right next to the treehouse. And it doesn't appear that this is just another angle, as we see the treehouse from a distance numerous times, and there is no grass or house anywhere in its area, let alone a full back yard with a grill.
Factual error: In his voiceover narration, Smalls says that Babe Ruth's "called shot" was made at Yankee Stadium in the bottom of the ninth inning in game 3 of the 1932 World Series. That play actually occurred in the top of the fifth inning at Wrigley Field. It's not just a character mistake; the event is too famous for him not to have heard or read about it growing up, especially since his stepfather is a Yankees fan.
Continuity mistake: At the end when Benny gets the ball and runs from the beast he hops the fence. The ball is in his right hand as he jumps. When the gang helps him up, he doesn't have the baseball in his right hand. As he's running through town the ball is back in his right hand.
Continuity mistake: A while into the chase scene at the end of the movie, the dog chases Benny through a wedding reception. A large cake, about four or five tiers high, is being carried by a couple of bakers. They lose their balance and set the cake down, but eventually it gets sent into the air. While going up, it stays whole, but the shot changes when it's coming down, and it has now broken apart into many small pieces.
Revealing mistake: When the kids are at the pool and Squints has just jumped in the deep end, the other kids go crazy and then you can see water on the camera.