Factual error: At the last shot of the Fourth of July game of the Sandlot, high-pressure sodium area lights can be seen in the distance on the right side of the screen. This type of lamp was not developed until 1964 (two years after when this movie is set). (00:44:06)
Other mistake: When Benny signs the autograph for the fake baseball he signs it Babe Ruthe with an 'E' on the end. This can be seen when Smalls put it back in his step dad's trophy room.
Suggested correction: He did though it's apparently supposed to happen like that as one of the boys says "Baby Ruthe?" when he's signing it and a comment is made about how bad it looks. Strange as it seems; this appears to have been done purposefully for the movie thus would not be a mistake if that's what the actor was directed to do.
Factual error: The film takes place in the summer of 1962. The scene where the boys go to the local pool to swim, in actuality this is during the civil rights movement when racial injustice against African Americans was at its peak - they would have never allowed a young black child to swim in an all white pool during this year and time.