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.
Our Father - S3-E1
Continuity mistake: When Dexter gets out of the car and talks to his sister, the strap on the camera keeps flipping back and forth. (00:52:45)
Sunshine and Frosty Swirl - S7-E2
Continuity mistake: When Wayne Randall is eating ice cream, the size/amount of ice cream on the cone changes as the camera pans back and forth between characters.
Revealing mistake: In the scene where Rudy's body is found by the police, you can see Rudy breathing and his pulse in his neck. (00:51:05)
Continuity mistake: The door to Dexter's apartment opens out toward the walkway. However, in all subsequent episodes the door opens into his apartment.
See-Through - S2-E4
Visible crew/equipment: When the wife of one of the victims invites Angel in, the silhouette of a camera man is reflected in the door when she closes it.
See-Through - S2-E4
Visible crew/equipment: As the camera pans to Dexter just before he jumps over the fence at the police station, the shadow of a boom mike is visible on the white wall.
Other mistake: While Dexter's searching for info on Travis Marshall on his laptop, he opens the DMV Records window with an image of Marshall's driver license, and in the closeup at the bottom of the license it reads "Operation of a motor vehicle constitutes consent to any sobiety test required by law," with the word "sobriety" missing the letter "r." Additionally, under the image of the license it reads "Issued: 10-12-2006", but on the actual license image it reads, "DOB: 11-24-1977, Issued: 09-23-1990, Expires: 09-23-2012." So that means Marshall was not even 13 yrs old when he got his license, and the license won't expire for 22 years. Also, when Brother Sam knocks on the door Dexter clicks back onto the MOA website, and if you care to pause the screen note the word "Exibit" is missing the letter "h." (00:18:50)
Other mistake: When Dexter is running Matt Chambers' fingerprint though his computer, he brings up four newspaper articles where the names Matt Brewster and Matt Rasmussen are used. In the final article, the headline reads "Matt Rasmussen of South Boston arrested", but the body of the article uses the name Matt Brewster exclusively. Other than the first few words, the text of the final article is identical to that of the previous one. (00:33:50)
Shrink Wrap - S1-E8
Continuity mistake: The Ice-Truck Killer is responding to Dexter's Craigslist submission. The reply-to e-mail address is frozenbarbie@hotmail.web, but when in the new window that popped up when the ITK hit "Reply" the address is frozenbarbie@hotmail.com.
Easy as Pie - S3-E7
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. (00:39:50)
There's Something About Harry - S2-E10
Continuity mistake: When Angel is drinking with Lila after finishing her apartment, the wine glass changes position throughout the scene.
Revealing mistake: The actor portraying a dead body noticeably moves her left leg while being turned over by Angel, Dexter and Masuko. (00:06:33)
Continuity mistake: When Dexter is killing Stan Beaudry, the drop of Beaudry's blood encompasses the width of the blood slide when the two pieces of the slide are pressed together, but then goes to being roughly half the size when the shot changes.
Make Your Own Kind of Music - S8-E9
Continuity mistake: Scene with Deb and Joey having lunch. Their water glasses switch between having ice and lemon wedges, to having no ice and a lemon slice floating at the top. (00:20:15)
Shrink Wrap - S1-E8
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. (00:05:30 - 00:06:00)
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.
Slack Tide - S4-E7
Other mistake: When Dexter shreds Laura Moser's file, he actually shreds a file for Estrella Carazo. (00:32:57)
Continuity mistake: Dexter kills Jimenez with a chainsaw and is wearing an apron. But he gets blood all over his shirt. He leaves in a hurry after getting a call from Rita about her door being unlocked and him having the only other key. When he shows up, there is no blood on the same shirt he was wearing.
Factual error: When Maria is looking at Miguel's file, we see his Florida Driver's License. The expires date is wrong. FL driver's licenses expire on the person's birthday, not on the day the license was issued.
Answer: Two reasons. First, as a cop he knows that there are lots of murderers walking around free due to flaws in the system so he uses Dexter's "urges" to right the wrongs in the system by taking out people who have no right to walk around free. Secondly, by taking him to a psychologist he runs the risk of having him committed and he doesn't want that for obvious reasons, so he figures it is best to channel his urges to what he considers a greater good.
The_Iceman
In addition to the earlier answer, in the last season (8th) of the show, we are introduced to Dr. Vogel, whom Harry had been discussing the situation with Dexter as well, and it is shown on-camera that Harry describes what Dexter has done (killing)... So, technically he did consult a psychologist/psychiatrist, just not having Dexter and her actually meet. But Dr. Vogel was aware of whom it was and how to intercede.