When the kids are running through the halls of the school, they run past a window that seemingly reveals it is dark outside. In actual fact, they are running past doors leading into classrooms, in which the lights are turned off because it is the weekend.
While Bender is crawling through the ceiling, you can see the chain for his wallet while he's crawling in both directions (flipped shot); the first scene his lips are moving (telling his joke), the second scene, they are not moving.
When Bender is staring at Allison while she bites her nails, his white shirt had been pulled low, exposing a part of his chest. In the next shot of him, seconds later, when he has obviously not moved, his white shirt has been pulled back up.
When Bender sticks the knife into the chair it is leaning slightly to the left side of the screen. The movie cuts back to Andy and back to Bender where Allison steals the knife that is now leaning to the right side of the screen.
When Bender is looking through Claire's stuff, he brushes his teeth with a makeup brush while looking in a pocket mirror. He puts down the brush on the table and looks in the mirror. He then sets the mirror on the table before tossing the brush onto the table a second time.
After Allison makes her crunchy sandwich, her soda has the straws making a perfect "V" . In the next shot with Bender going through Brian's food, Allison is seen in the background with her straws pointing to just one side. While Bender gives his 'life at the Johnson house' story the camera shows Allison once with her straws back to their original "V" position, and then again after Bender jumps up onto the staircase just subsequent to his cigar story.
During the beginning scene where the kids are being dropped off, you can see a dog in Emilio Estevez's car after Anthony Micheal Hall exits his car. When the shot switches to Emilo's conversation with his father, the dog is nowhere in the vehicle.
When Mr. Vernon is chasing John Bender around the school, in one of the shots where Bender turns the corner, there is a large stand-up bulletin board and a small table to the very bottom right of the screen. When Mr. Vernon turns this same corner, the small table is gone.
Before Allison tells Andrew about how her parents ignore her, her left eye make-up is smeared. While Andrew is questioning what's wrong, it isn't smeared as much. When there is a close-up of Allison's face, it is back to normal.
When Mr. Vernon comes in and asks why is the door closed, you can see the chair he later uses to keep the door open, near the corner of the wall. When he goes back to get it, the chair has moved a foot to the right.
When Vernon is handing out the paper and pencils, Brian can be seen from behind and his hat and gloves are next to each other on the table; after Bender makes the Barry Manilow comment the next two shots of Brian have the gloves on the hat and then back on the table next to it.
When Principal Vernon is caught looking through confidential files by Carl the janitor, the position of Carl's arm as the two discuss the situation changes from on top of the cabinet to inside the drawer with each cut.
When Brian is talking about the banquet at the end of his physics club, the camera cuts to Allison smiling at him while she's drawing and she's sitting up. When the camera cuts to Claire a second later, Allison is resting her head on the desk.
Principal Vernon brings Bender into the janitors closet after he is caught in the gym, and while in the closet he insists that Bender punches him by pointing at his own chin. When camera angles change you will see that his hands alternate while pointing at himself, from left hand to right hand.
In the beginning of the movie Bender has two matching shoes, in the gym he has one white shoe and one black. In that same scene he loses one shoe, but a scene later he has two matching shoes again. This is because he's stolen a pair of shoes and only puts one on, then later the other.
Throughout the movie it appears that Bender is wearing Claire's diamond earring in his left ear, even though she doesn't give it to him until the very end of the film. However, it is actually an ear cuff that is worn anywhere above the earlobe on the outer part of the ear. It is clearly seen when he puts on her earring.
In the opening sequence, there is a newspaper with a wrestling article featuring Andrew "Clarke". In the credits, Andy's last name is spelled "Clark". It is not uncommon for school papers, written and edited by students, to have typographical errors.
When Andrew is doing his crazy dance scene in the Library after getting high he jumps through the Georgia State Flag (the Confederate Battle Flag) a couple times.The High School is in Illinois though. The flag may have been in there for historical purposes; the Civil War and the Confederacy are common study material in a high school history curriculum.
The reasons Bender, Andy and Brian were in detention (pulling the fire alarm, assault and having a gun at school respectively) would typically warrant them being expelled at most schools. In a modern school this is certainly true, but the film came out in 1985, when attitudes towards these activities would have been strict, but more lax than those of the modern era. Additionally, Brian brought a flare gun, not a handgun that could have killed or injured someone.
On a far wall, in the school library, there is a magazine rack and bookshelf that reads "Librarians Selections." This is grammatically incorrect, as it is possessive and should read "Librarian's Selections."