It goes from pitch black to twilight and back again several times during the scenes after the baseball game and before the party at the moon tower.
At the end of the movie, Mitch Kramer is making out with the Sophomore girl next to her car and the sun has already come up. When he arrives at home after that has happened, it is still dark outside.
When Dawson, Pink, and Mitch are cruising around with Pickford, Pink goes to pick up a trash can, but its Dawson's arm that grabs the can. Then Pink is shown in his windup.
Near the end of the movie when they are smoking a joint on the football field, the long shot shows them all sitting about 20 yards from a large dead patch of grass, but in the close-up, Slater's head is almost in the dead spot.
When Slater tells Pickford to crank up Foghat's "I Just Want to Make Love to You" he's in the back seat on the drivers side. Two scenes later when he says "check ya later" Slater is in the back seat on the passenger side.
Many of the male characters hair it too long. Throughout the 70's all Texas schools had dress codes that did not allow boys hair to be long enough to touch their collars. Also, hazing (in schools that allowed it) happened during the first week of a new school year, not the last day.
At the end of the school day they are all driving around about an hour or two later, and it's already pretty dark outside. It's late spring/early summer, and it shouldn't be that dark at 5pm. It doesn't get dark like that until around 8pm.
On the "Countdown '76" display in the history teacher's classroom, then-U.S. Senator and potential Presidential candidate Lloyd Bentsen's name is misspelled "Benson."
The motorcyclist's shirt. While a similar design was used later by The Misfits, the design (if indeed it's the same skull) ultimately comes from the movie The Crimson Ghost (1946).
While O'Bannion flunked, the tassel on his mirror could have been purchased before that was known.
When Carl, Hirschfelder, and Tommy are walking away from a neighborhood street corner. In the background O'Bannion is approaching the corner. Before he passes the tree his duster is an orange color. After he passes the tree, it is a primer gray. The orange hue is being projected onto the car from the porch light of the house.
One girl's reference to "Madonna" is to the Medieval Catholic ideal, not the pop singer Madonna Ciccone who first appeared on the scene in the 1980s.
During the freshman hazing, Simone is wearing tube socks with tennis shoes. Another girl is seen earlier wearing black pumps with no socks that some have confused with her.
When Pink, Don, Mel and Benny are in the back of the truck, examining Benny's cracked paddle, O'Bannion pops out early from behind the building at the far right (widescreen editions), then quickly ducks back out of sight and emerges on cue to deliver his lines.
When Mitch's sister, Julie and Mitch are talking at the party, Mitch's sister pats Julie on the back and she stumbles, revealing her "mark" a yellow "t" marked out in tape on the grass so she knows where to stand for the shot.
When Pink is throwing trash cans out of the car you can see the wire attached to the car from the trailer it's riding on, also the tires aren't moving.
In the final scene where the gang is driving down the highway in Wooderson's Chevelle, the shots looking through the windshield into the car show the cowl induction flapper of the Chevelle hood open, if the motor was running the flapper would be closed (unless the engine was under full acceleration). The next shot through the windshield looking down the road shows it being closed.
In the opening scene while the GTO is cruising through the school parking lot the car is an early-'70s Camaro with the wheels from a mid-'80s Camaro.
The seniors are driving around, chasing freshmen to paddle, where at one point they gun the engine to increase speed, start swerving at the boys before coming abruptly to a stop. Moments later, one of them pops open the trunk to reveal a nearly full trunk of glass bottles of beer (easily more than 6 cases), yet none of them are broken. Having defied the laws of physics and motion, they're all in pristine condition.
In the coaches clean-living pledge sheet the word "committed" is spelled "commited"
Carl Burnett is shown wearing a t-shirt with the KISS album cover for "Rock and Roll Over" which was not released until November 1976.
Throughout the film, the cast members are drinking generic cans and bottles of beer yet at the gas station and arcade there are present day (early 90's) signs for Budweiser and Bud Light. Bud Light wasn't made until 1982.
When Mitch goes to purchase beer at the convenience store, a Coke dispenser with a logo first introduced in 1985 is visible.
In the driving scene that ends the film, the car that the main characters are in passes an Oldsmobile Delta 88 that was made after 1980, yet the movie takes place in 1976.
When Hirschfelder, Carl and Tommy were walking together just before Carl throws the beer bottle, there is an early '80s Datsun 510 parked at the side of the street.
When Carl's mom confronts O'Bannion with the shotgun, she can be seen saying "Get the f*** off my property," but the entire line has been dubbed over with "hell" replacing the more extreme profanity.
When O'Bannion, after dumping the paint, drives his car over a patch of grass, the tires squeal as if on pavement.
When driving to the outdoor party Wooderson and Benny are yelling at each other but the words don't match their mouths.
When O'Bannion jumps out of the car to chase Mitch and Carl, you will hear the sounds of both cars speeding away, but if you actually watch behind O'Bannion, you will see that Benny's truck is still there, even after both tire squeal sounds can be heard, even after the rumbling sounds of the truck starts.
When buying the beer and chatting with the shop worker you don't hear the shop worker crumple the bag to roll the top portion but when you see it the beer is in the bag all ready to go.
When Simone says, "I did it when I was a freshman, and you'll do it when you're seniors. but you're doing great. Now fry like bacon, you little freshman piggies. Fry!" you can see a reflection in the window of a person crouching down signaling two extras to walk past in the background.
Outside of the Emporium, when Slater asks Mitch "You cool man?," in the background behind Slater the crane of the lighting rig across the street, (used to illuminate the parking lot), can be seen.
Reflections visible in the car door when the guys pull up to buy beer.
After pouring the paint and jumping off of the roof, when they get into the car, you can see the shadow of the camera men in the grass.
When Hirschfelder and Tommy are jumping from the roof after dumping paint on O'Bannion, the shadow of one of the stunt doubles can be seen even after the actors have jumped up to make their way down to street level.
Reflection in classroom window at the beginning of the movie.
Boom mike reflection in window behind "Mike" and "Tony" when "Pink" first encounters them in the hallway. Then as "Tony" and "Mike" walk a little down the hall the boom mike pole appears in the top right side of the screen a few times.
Wooderson tells Clint that his '70 SS Chevelle has been bored over 0.030, and other upgrades and claims 390 horsepower. The '70 the 454 SS Chevelle came equipped with either a LS6 engine which was factory rated at 450 horsepower or LS5 454 rated at 360 horsepower. The engine modifications that Wooderson refers to would be less than what an LS6 engine already had so it's likely it would have been the LS5 version, which is actually more rare. The modifications would have increased the horsepower and the 390 horsepower claimed was easily achievable, if not more.
During the scene in Pickfords GTO when "I Just Wanna Make Love to You" starts playing, someone says "Crank it up!" Pickford turns the right radio knob and the volume goes up. In actuality the right knob is for station selection. The left knob is the on/off and volume control.
When Pink is leaving the baseball game, the old man asks "is this arm is ready to throw about 2000 yards" while grabbing Pink's left arm. Later on, at the football field, Pink throws the car keys to Wooderson with his right arm.