Factual error: In a couple of scenes, most notably the one where Graysmith drives his kid to school, there are newer cars in the background than the year it is supposed to be. In this scene the car is a 1971 Chrysler, when the scene takes place in 1969.
Continuity mistake: In the scene where Graysmith goes to Vaughn's house, his jacket is soaked from the rain. Moments later he is sitting in a chair, and we can see his jacket is dry. Not enough time passed in the film for the jacket to dry.
Continuity mistake: During the scene when Graysmith and Toschi are having lunch, the burger Toschi is holding, and taking bites from, has lettuce hanging out of the bun in some shots, but not in others. Most of the shots from behind him show the lettuce, while most, but not all, the shots from the front don't have lettuce showing. He does put the burger on the plate a couple of times, but the lettuce is still inconsistent in what is meant to be a continuous action.
Factual error: The outdoor night scenes are illuminated by high-pressure sodium vapor streetlights, which did not come into use until the mid-1980s. Streetlights of the 1970s used mercury vapor lamps, which give a quite different bluish-white light.