Continuity mistake: When Robert Graysmith is speaking with Sherwood in Sherwood's garden, Robert has his arms crossed. The watch on Robert's left hand changes multiple times between shots. (01:58:25 - 01:59:05)
Continuity mistake: In the shot where Robert Graysmith goes to see Paul Avery on his boat, the pong game on the T.V. in the background goes from 15:9 to 14:9 back to 15:9 between shots.
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.
Continuity mistake: When Robert Graysmith visits Paul Avery on the houseboat and Avery sits on the couch he is barefoot but his foot has a sandal on in some shots. He has the sandal on when he rises. (01:43:10 - 01:44:50)
Continuity mistake: After Graysmith slams the phone down in his apartment and his wife enters, the phone receiver wire changes position a few times before she leaves.
Continuity mistake: Early when the two are going to park, the girl is driving a 2-door Corvair. But the interior shots of the car are of a 4-door Corvair.
Chosen answer: Scorpio is loosely based on the Zodiac killer, yes. However, "Zodiac" is made to portray the hunt for the real-life Zodiac killer (as accurately as possible), with only a few ficticious elements thrown in for the sake of story-telling, while "Dirty Harry" is pure fiction.
Twotall