Corrected entry: Some of the computers shown in the scenes at the newspaper, set in the 1970's, are a newer style than what was available at the time. They look like they are from the 1980's.
Corrected entry: When Darlene is parking her car in a "more quiet place", you can see the tire marks of where she is going to park, from previous takes.
Corrected entry: Toward the end of the movie there is the scene where Robert Graysmith goes to Arthur Leigh Allen's work place and stares at him for a long time. In this scene you can see Leigh's name tag, but his name is spelled "Lee" not "Leigh". This could be considered not a mistake, however in an earlier scene we see case files with his name specifically spelled "Leigh".