(around 21 min.) When Jeff returns to school in 1978, there's a poster for a Science Club. "Science" is misspelt as "Sciece".
Around the 31-minute mark, the word "Chapters" on the blackboard is misspelled as "CHAPHTERS".
Toward the end of the movie when Dahmer gets in his VW, and turns on the Radio the DJ mentioned the Michael Stanley Band playing the night before. The Michael Stanley did not have a concert in Cleveland in June of 1978. The Michael Stanley Band had NO tours dates anywhere or anytime in June of 1978.
When trying to set up a meeting with Mondale's aide the characters say they are on Constitutional. It is Constitution Ave.
(around 36 min.) After Jeff gets his picture taken with a grim expression, he momentarily looks at the camera as he laughs with his friends.
Near the start of the film, someone wears a Stagecrafters shirt - which is an acting group run by the mother of the Dave Dahmer actor.
At about 36 min a green blackboard is seen with 'Read Machiavelli's The Prince chapters 1 and 2' Chapters is misspelled as chaphters.
When Jeff gets kicked out of a supermarket for goofing off, several modern cars pass by from outside.
While Jeff and his friends drive down to the mall, present day vehicles are visible behind them.
On a couple occasions, Derf is seen drawing with a Sakira Pigma Micron pen. This particular artistic lining pen wasn't invented until the mid 80's; yet the movie takes place in 1978. Another time Derf is sketching with a Faber-Castell Pitt Artist pen, another drawing pen that wasn't available in that same time period.
The song "5-7-0-5" by City Boy plays in the background of a school scene early in the movie, but Dahmer graduated from high school in the spring of 1978, while the song did not come out until Fall 1978.
Two boys on bikes mention that they are hurrying home because they don't want to miss Monty Python as they return home in the late afternoon. Monty Python's Flying Circus, a British comedy show, was always shown on PBS late at night because of it's adult content.
When Jeff, his brother and mother (Joyce) arrive at the courthouse, the cameraman is momentarily reflected in Joyce's sunglasses.
(around 1 hr. and 12 min.) The camera's reflection can be seen behind the car, after Jeff and his friends park.
When Dahmer is talking to Bridget in the hallway about prom, the boom is visible in the reflection in the door behind Jeffery.