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.
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.