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.
Continuity mistake: A windshield wiper is visible on Milner's coupe in only three shots. These occur when Carole gets into the coupe from the Studebaker and shortly after, when Milner is pulled over for a defective license plate light. All other shots of the coupe show only the stub of the wiper mechanism.
Revealing mistake: At one point John is complaining about the Beach Boys playing on his car radio, he reaches to the center console and motions turning the radio off. In a few other shots though, we see that the car appears to have no radio at all, the center console seems to have a large tachometer and speedometer there, but that's all.
Other mistake: When nerdy guy (glasses) enters the pharmacy and goes to the counter, he nervously requests chewing gum, hair comb, candy bar, pan, batteries, a pint, and beef jerky, which the clerk just turns around and then magically puts into a bag, including the booze, when most of those items except the booze should be available on the shelves behind them.
Other mistake: When Carol leaves her car full of girlfriends to get into John Milner's yellow coupe, she does not reach for his obvious door handle, but instead puts her hand through the open window and uses the inside release. She could not possibly know the outside handle would not work.
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.
Continuity mistake: When Wolfman Jack was eating a popsicle, it had a cellophane wrapper, in those days they only had paper wrappers.