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 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: 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.