Factual error: When Dexter and Doakes are looking at the severed foot wearing a soccer cleat, Dexter states, "The foot was severed between the tarsus and the metatarsus." However, the foot was severed proximal to the tarsus, through the tibia and fibula. The cut was easily too high to be between the tarsus and metatarsus.
Factual error: When Dexter is in the hospital as a child, the doctor says that Dexter has a very rare blood type, AB negative. Although it's very rare, the people with AB- can receive donor blood with A-, B-, AB- and 0-. AB- is very good blood-type when you need to receive blood. While it is true that hospitals normally have to use AB- because of the complexity of other proteins (and to prevent adverse reactions), in case of an emergency, like Dexter's, and a shortage, a hospital will use a blood type close enough to not likely cause a reaction. In this situation, the hospital would give Dexter O-, B-, or A- instead as blood loss would normally kill a patient long before a suitable donor could be located, called in, admitted to the hospital, and have the transfusion set up.
Factual error: In the beginning while Dexter is on his boat in Miami, you suddenly see mountains in the background, of which there are none in Miami. Also the palms are too high, indicating they are from California. And the worst setting mistake is the Florida tags in front of the cars. Only commercial vehicles have front tags.
Continuity mistake: When Doakes is about to be killed by Guerrero, the police save him by shooting Guerrero, giving him a bullet wound on his right shoulder. He is also cuffed on the ground to be arrested. However, when he is pulled up, Guerrero does not have a bullet wound anymore.
Continuity mistake: When Dexter and Rita are having dinner at Miguel's house, his wife drops a glass. She sets 2 other glasses on the island in the kitchen. After a cut to another shot the 2 glasses are now on the counter by the sink, before she bends down and picks up the glass she dropped.
Continuity mistake: When Doakes asks Deb "what took her so long" arriving at the first crime scene, Masuka is stood next to him. However, over the course of the conversation Masuka is seen in the background talking to another group inside the house, and in the next shot he is standing back next to Doakes. At the end of the conversation he walks over to the group inside the house.
Continuity mistake: In Season 6, Episode 10, Dexter and Harry are in the motel room where Travis was staying (timestamp 1). Dexter decides to post on Gellar's blog (impersonating Gellar), and the screen on the computer shows the post date as Sunday, November 24, preceded by a Sunday, November 20 post. Aren't Sundays supposed to be a week apart? Then in episode 12 (timestamp 2), Travis is in Dexter's apartment and sees the poster for the Noah's Ark school play. The date posted for the play is Saturday, October 8, ie. in the past compared to the earlier episode. Plus If November 24 were on a Sunday, then October 8 should have been on a Tuesday.
Continuity mistake: When Doakes is carrying the packs of cocaine out of the cabin, the position of his arms and the cocaine varies wildly from shot to shot. Notable examples are that the cocaine goes from being arranged in neat, accurate stacks, to being haphazardly and irregularly stacked, and back to neat again. His grip on the cocaine also varies a lot; he starts out with straight arms, then suddenly has a 90-degree bend at the elbows, and back to straight again, all depending om camera angle. All of these changes happen too fast to have been natural, and Doakes is on camera the whole time.