The Gaslight Addition neighborhood is actually the Country Walk neighborhood in Savannah, GA. The earlier scenes of the movie are set in 1970, but the houses used in the movie are architecturally characteristic of and and were in fact built in 1990.
Badfinger's song "No Matter What" had not been released in the U.S. by the summer of 1970. The song had been recorded in London in April 1970 but was not considered for American release until a U.S.-based Apple executive heard the song. The single was released in the U.S. in November, 1970. The LP the song appears on, "No Dice", was also released in the U.S. November 1970.
At the softball game, one of the girls calls a boy "nerd-o". Although the term "nerd" had been around for years before, it didn't become popularized until its use in the sitcom "Happy Days", which aired 4 year later than the time depicted in this movie.
In the summer of 1970, the girls ride their bikes under a railroad trestle on which gray tanker cars are passing with the Norfolk Southern Corp. logo (leaning "NS" with horizontal stripes), but that company did not exist until 1982.
When the girls are painting the garage and the Wormer brothers walk by, one of them insults Chrissy and calls her "lard butt," though it's clear from his mouth movements that he was actually saying "lard ass."
Sam is seen with a map of Indiana in her car. Considering she is going to her childhood home and the very house her friend lived in as a kid, why does she need a map? No one needs a map to find their old home town.
In the movie they say that their home town is Shelby, Indiana. Shelby, Indiana is in the north western part of Indiana. They went to Greenfield, Indiana to go to the library, on their bikes. That is impossible: they are approx 150 miles apart.
On the railroad bridge, at the beginning of the scene where they go to see the "Mystic Advisor", it says ACL RR for Atlantic Coast Line Railroad. This railroad only ran in the south east United States. The movie is supposed to be in Shelby, Indiana. They would have had to ride their bikes to, at least, Winston-Salem, North Carolina to see a bridge like this.
The girls are looking into how "Dear Johnny" died. They are searching many large books, the bound newspapers from the past, but they know the date Dear Johnny died. They only need to look in the papers a published around that date.
When the girls are looking for articles from 1945 in the newspaper Roberta finds an article about her mother's death. Her mother would have had to have died after 1958, therefore Roberta shouldn't have seen the article unless she wasn't actually searching in 1945 like her friends were.