In order to establish the close bond between Smalls and Benny, the director had Tom Guiry and Mike Vitar meet and rehearse together weeks before the rest of the kids showed up to film. It worked so well that the other kids genuinely believed the two actors had been friends for a long time.
When the boys are in the pool watching Wendy Peffercorn, the dialog: "She don't know what she's doing" and the answer "Yes she does, she knows exactly what she's doing" is the same as in a similar scene in Cool Hand Luke (1967).
When the narrator is describing how everyone grew up towards the end, he says that Bertram (Grant Gelt) "got really into the sixties, and no one ever heard from him again". In real life, Gelt would go on to land a part in The '60s (1999).
The name of the other park the sandlot kids play the little league team at was the L.C. Romney Baseball Park (on the scoreboard at 47:40 and 48:04). L. C. Romney was a commissioner of Salt Lake City and a democratic candidate for Governor of Utah in 1956. The film was shot in Utah.
Mr. Mertle (James Earl Jones) shows the boys a photo of himself with Babe Ruth and Lou Gehrig. Of course it is a retouched photo. Jones' head was put on the body of Hall Of Fame slugger Jimmie Foxx who was the real player pictured with Ruth and Gehrig. The real photo can be seen in many baseball books.
The Vincent Drug store that is seen when Smalls first goes to the "Sandlot" was also used five years earlier in Halloween 4: The Return of Michael Myers (1988) when Rachel and her friend take Jamie to pick out a costume. It was also used the next year in Halloween 5 (1989): The Revenge of Michael Myers in the scene where the Man in Black steps off a bus and makes his way through town.