Factual error: In one scene a parked car is sitting along the street - it is a '68 Chevy, it has sidelights. Yet the film is set in 1962.
Factual error: In one scene, Milner does a U-turn and you see a postal box on the curb. The logo on the box is the current white eagle drawing, but that was not used until well after 1962.
Factual error: Shortly after Milner picks up Carol they make a right hand turn off the main street. There is a White 1971 Toyota Corolla at the intersection directly in front of a white 1960 Pontiac. The movie takes place in 1962.
Factual error: The Pharaohs are breaking into the pinball machines for gas money -- machines that weren't invented at the time the movie was supposed to take place. The Ball Park game was made by Williams in 1968 and the Vampire and Skyrocket games were made by Bally in 1971.
Factual error: While Falfa is singing "Some Enchanted Evening" to Laurie, the radio commercial in the background is by Arnie "Woo Woo" Ginsberg, touting the Gingsburger for Adventure Car Hop in Saugus, Massachusetts. This could not have been received by Falfa's car radio in California, since the commercial was actually available via WMEX in Boston, Massachusetts.
Factual error: When the cruising starts at the beginning, where the yellow Chevy has its hood open, parked in front of it is a gold 1968 Chevy. Several seconds later, a '56 and '50 Chevy are next to each other and the '56 has a California blue plate, 189 FJY. Blue plates weren't issued until 1969. Also, when Toad pulls away with Debbie, the long shot has a '55 Chevy with black plates go right in front of the camera. Black plates didn't come out until 1963. When the axle is ripped from the police car, the marquee on State theater in the background says Dementia 13 is playing. That didn't come out until 1963. (00:10:00)
Factual error: When John Milner is getting the ticket from the cop, he pulls over in front of a movie theater. The poster for the movie "Cabaret" is clearly visible in the background several times. You probably wouldn't know this unless you are old enough to have seen the movie in a theater, as "Cabaret" was in theaters only a year or so before Graffiti came out, so isn't appropriate for the time AG is set.
Factual error: Although the movie takes place in 1962, at least one car is wearing a black background California license plate -- which were not issued until 1963.