Continuity mistake: When Toad turns the 58 Impala around to pick up Debbie. She walks past the same Blue Corvair twice. She then steps from the curb to talk to Toad. The Corvair is parked next to the curb. When they switch to the long shot the Corvair has been replaced by a gold coloured Buick. When they drive away the Corvair reappears in its original position.
Visible crew/equipment: As Milner and Carol get out of his hot-rod to vandalize the girls' stopped car, a bearded man with a mustache can be seen in the car window's reflection as it's being rolled up. The man is wearing a white long-sleeved shirt with what appears to be a maroon sweater-vest. I believe the man to be George Lucas, as he appears to be directing in an upward motion with his left arm.
Continuity mistake: After Carole is hit with the water balloon thrown from a passing car, she tells John Milner to catch them at the light, where he's to flatten their tyres while she sprays the windshield with shaving cream. Carole sprays the front windshield, then starts to go to the back of the car to spray the back window. The front windshield is clean, even though Carole just emptied a can of shaving cream onto it.
Continuity mistake: The girls - whose car tires are about to be flattened and windows about to be sprayed with shaving cream - roll up their passenger window twice during the same brief scene. Once as Milner and Carol are getting out of his hot-rod, and again as John and Carol approach the car.
Deliberate mistake: The Studebaker in which Carol is first riding bears the license number JPM 351, which interestingly enough, is not only the same license number that Steve's 58 Chevy has, but is also worn by the car Curt is sitting on when he first meets the Pharaohs. To top it off, the Edsel driven by Cindy Henderson has plates reading PRC 596, which are also used by a silver 57 Chevy and a red pickup.
Continuity mistake: During the first set of cruise scenes, you can see a shot of a yellow '56 Chevy parked on the opposite side of the street with the hood raised and a coupe of guys working on the engine (just after a purple-ish '40 Ford rolls past). In the very next shot, you can see the same car for a very brief moment, this time with the hood down. A few shots later, you see the same yellow '56 yet again, but this time being driven. Finally, in the very next shot, Milner drives past it yet again, still parked on the side of the street, with the hood down.
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: In the race scene between Milner and Falfa, you can see exhaust smoke coming from Milner's car, but in a previous scene the station attendant mentioned Milner taking the header plugs off. By taking the header plugs off, his exhaust is running from the headers and you wouldn't see exhaust coming from the tail pipes.
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.
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.