Continuity mistake: When the family is getting into the car to go to the wedding one grandmother says to the other "let me in". The grandmother gets out to let the other grandmother in but the first grandmother never gets back in the car, she just pretends to. You can see her hiding behind the cars as it backs down the drive. Both grandmothers are in the car when they see Long Duk Dong on the lawn.
Other mistake: The chunk of hair that Ted has in the car is in a ponytail, and much longer than the hair cut off in the door.
Other mistake: Even though the door was locked on the half-built car, there was nothing to stop it from opening because the pillar (and mating piece for the latch) was missing.
Other mistake: When Long Duk Dong pulls up in front of Samantha's house to drop her off before going to Jake's party, there is a drive-in restaurant tray attached to the passenger-side door of the car. On top of the tray are paper Coke cups and burger wrappers. These had supposedly remained on a tray outside of the car even though Dong was careening around at high speeds. When Samantha gets out of the car and slams the door shut, the wrappers and cups still do not move.
Audio problem: When the geek and the prom queen are cruising down the road in the Rolls Royce convertible, the drunken prom queen says to the geek "Looks like rain, better put the top up." However, her mouth movements do not match the words.
Revealing mistake: When Carolyn and the Geek are in the parking lot across from her church, Carolyn gets some help as she climbs over the side and out of the Rolls-Royce - as opposed to using the door. As she makes her way over the side, you can see that she steps onto a step-stool or similar above-ground block before stepping down once more with her next step onto the pavement. There is no such step on the car itself.
Continuity mistake: In the opening shot of Jake's party, a girl wearing a light, multi-colored dress is sitting on the back of the sofa talking to a guy in a blue t-shirt. In the close-up of Jake, as he enters the room (with a disgusted look on his face) the girl is now sitting at the front of the sofa chatting with the prom queen, who was not there before.
Continuity mistake: The guy with the red Star Trek glasses is standing behind Sam and Randy as they walk in at the beginning of the dance, and he is staring at the scoliosis girl (Joan Cusack). Then in the next shot, he is standing with the geeks on the other side of the gym. You don't see him walk away at any time during the sequence when Sam and Randy are talking. How did he get over there?
Deliberate mistake: When the geek and the prom queen are driving along in the Rolls-Royce rebel yell scene, the close-up shot of them through the windshield shows a missing rear-view mirror. It was there when the geek pulled out of Jake's garage and it reappears later when the Rolls is parked in front of the house where John Cusack's character lives. This was probably a deliberate mistake by John Hughes so that he could get a better shot of the prom queen feeding birth control pills to Anthony Michael Hall.
Audio problem: When Samantha is lying on the bed discussing her love life with her sister, she is also scanning the pages of a bridal catalog. As the scene flips back and forth between Sam and her sister, there are times when the pages in the catalog change without hearing Sam actually turn the pages.
Continuity mistake: When Samantha and Randy are getting on the bus, they both scan past the juvenile carnage to the back of the bus where the adolescent band is practicing. The geek is nowhere to be seen. However, when they take their seats, the geek immediately appears behind Samantha at the back of the bus leaning on a trombone case.
Continuity mistake: When Long Duk Dong comes speeding out of the parking lot after the dance, there is only one other person in the car with him and he is not wearing any sunglasses. When the shot immediately changes to a close-up through the windshield, there are now two other people (Samantha and the sexy girlfriend) in the car and The Donger is wearing sunglasses.
Other mistake: When Ted and Sam are in the half built car on the night of the dance she remarks that she felt how much he liked her on her leg, but his crotch didn't go anywhere near her leg.
Continuity mistake: At the party we see Long Duk Dong and sexy girlfriend crash Grandpa's car into the front yard with a drive-in window tray attached to the driver's window. The scene prior to this was them at a drive-in restaurant eating and causing a ruckus, but this scene was cut from the final film. This is a typical continuity error when prior scenes are cut from the final film.