When Mary Elizabeth opens the bottle of wine, she smells it and says the wine has to breathe. When Charlie holds the bottle, it can be seen that the cork is still in the bottle.
During Homecoming, Sam's dress has clear straps. These straps, however, are non-existent in the later scenes in this dress, most notably in the tunnel scene.
During a closeup of Sam and Patrick doing the "Living Room Routine" at the dance, Charlie's face briefly can be seen entering the left side of the frame, before he'd even stepped onto the dance floor. The shot immediately cuts to Charlie up against the brick wall, and he steps forward to join Sam and Patrick.
When Charlie and Sam talk outside following the cafeteria fight, Sam's hair repeatedly switches between in front and behind her right ear.
After Patrick is tripped by one of Brad's friends in the cafeteria, the bottom of his shirt is tucked under his arm. After he says, "Your pet ape just tripped me", his shirt was back to normal. In the next shot, it's tucked under his arm again.
When Sam holds up her SAT results, she states she received a 1210. The paper does say her total score is 1210, yet the Verbal and Math scores are 550 and 460, totaling only 1010.
When Charlie is drinking the milkshake made by Sam, the contents in the glass do not move at all as he moves it around when talking to people.
In all of the cafeteria scenes, the food pyramid posters that can be seen on the walls are the updated 2011 versions, not the ones they would have had in 1991 and 1992.
Whenever Charlie, Patrick, and Sam go through the tunnel into the city, a road sign reads "Consol Center". The Pittsburgh Consol Center was built in 2008.
If the movie takes place during the 1991-1992 school year, the song "Low" by Cracker that plays as they enter Charlie's first party wasn't recorded or released until 1993.
If the movie does indeed take place in the 1991-1992 school year as the book would suggest, during Bob's party the song "Here" by Pavement is played, which was not released until April 1992.
During the end of the first semester, Patrick tells his shop teacher that if he fails, he has to retake the class again next semester, so his teacher gives him a C-. Though Patrick passed, he is seen taking the class in his final days of second semester.
Mr. Anderson claims that it was Charles Dickens who left a character literally hanging by his fingers in a serialized story, thus contributing to the term "cliffhanger." In fact, the author was Thomas Hardy, in his 1873 serial novel A Pair of Blue Eyes.
On the back wall during Charlie's first English class, Emily Dickinson is misspelled as "Emily Dickenson".
Sam is regarded as a music fanatic, particularly in regards to artists belonging to the glam and new wave genres. Anyone with any significant exposure to this music undoubtedly would recognize a David Bowie song by his voice alone. It is not believable that it would have taken Sam more than a school year to identify the tunnel song as Bowie's "Heroes".