When Dahmer is purchasing alcohol in the convenient store, the "WE CARD" sticker on the freezer to the right says the "born on this date" year to purchase alcohol is 1990.
The number on Jeffrey Dahmer's door in the film is "214" he in fact lived in apartment 213.
When Dahmer is watching the Chicago Pride Parade go by, there is a woman in the front row of spectators recording the event on her smart phone. This was supposed to be 1991.