Factual error: In a scene in 1980, Brian May is teaching everyone the beat for his new song "We Will Rock You." However that song was released in 1977.
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.
Factual error: The shop owner shows him a tape of the Joshua Tree by U2 and says it is the best concert he has seen, but the Joshua Tree wasn't released until 1987.
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.
Suggested correction: Euronymous' apartment door opens outward into the hallway, at least in the final scene it does when he's trying to get away from Varg.