Factual error: Towards the end of the movie when Ally is in her hallway looking at Jack's old concert tour posters hanging on the wall, the poster closest to the camera says "Washington State Fair" and a 2011 date. Unfortunately, that fair did not get that name until 2013. Before that, it was called "The Puyallup Fair" (officially named the "Western Washington Fair"). Locals still call it "The Puyallup Fair" to this day.
Other mistake: During the murder in the stairwell, an old woman opens her apartment door outward, and Varg kicks it shut. Apartment doors always open inward, as did Euronymous' apartment in the same building.