Our Father - S3-E1
Continuity mistake: When Dexter and Vince are next to the dead girl, in the side shot, the tied string from her top is visible on her back and her hair is to the side. But in the overhead shot of her, her hair is covering the tied string and spread more over her back.
Our Father - S3-E1
Other mistake: When Debra is talking about her new haircut, she states that she has changed her hair "for the first time since she was 8." However in various flashbacks throughout the show she has drastically different hairstyles from her 'usual'.
The Lion Sleeps Tonight - S3-E3
Continuity mistake: A man in a blue t-shirt and holding two beer cups is seen walking in front across Dexter and Miguel. In the next shot the same man is seen walking behind them. (00:15:40)
The Lion Sleeps Tonight - S3-E3
Visible crew/equipment: While at the cemetery a reflector (to brighten the actors' faces) is regularly visible in the DA's glasses.
Revealing mistake: The dead body appears to be moving its eyes. (00:10:05)
Continuity mistake: Deborah walks out with the yellow shirted man in front of her. Next shot after Quin leaves, the same man re-exits the room. (00:14:00)
Continuity mistake: When Dexter leaves on his boat there are fishing poles behind his seat, but when he turns back around and you see him going away on the boat they have gone.
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)
Continuity mistake: In the scene where Dexter is playing a video game on Miguel's office computer, Miguel is shown to close the game down. But in the next shot the game is visible on the screen before disappearing again in the shot afterwards. Additionally, the video game shown is Halo 3, which is only available on Xbox 360. When Miguel 'closes' the game, he is actually alt-tabbing from a "TWVidpla" window to "Plaindesktop.bmp," and a laptop battery indicator is visible. (00:12:55)
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.
Do You Take Dexter Morgan? - S3-E12
Other mistake: The marriage certificate for Rita's first marriage shows her date of birth as 04-19-1989 and Dexter states she married at 16. The marriage date is 08-16-1989, when she would have been 4 months old. She also would only be 17 at the start of the series (2006) if she was born in 1989. (00:13:45)
Do You Take Dexter Morgan? - S3-E12
Other mistake: Debra is checking her email for the traffic cam photo of George King. The sender is labeled "Miami Metro Robbery Div." and the subject is "Photo of George King." However, after opening the email, the send is then a malformed email address of Det.Robertson.Robbery|MiamiMetroPolice.web with a recipient address of Debra.Morgan.Homicide|MiamiMetroPolice.wed (wed, not web), with a subject of "B. O. L. O. Match - George King." (00:26:38)
Do You Take Dexter Morgan? - S3-E12
Character mistake: The petition for dissolution of marriage was filed on Jan 6, 1990 which is less than 5 months of marriage. However, the duration of the marriage says it's been 6 months.
Revealing mistake: When Dexter is retrieving a blood sample after cutting Benny Gomez's cheek, there is no blood in the dropper and he puts no blood on the actual slide. (00:49:50)
Revealing mistake: When Rita confronts Dexter about his accident being more than minor Rita turns the buggy with the baby in around to leave and it is clear the baby is fake. (00:38:20)
Continuity mistake: When Dexter and Masuka are discussing wanting to go home Dexter throws a can toward the bin and misses, landing by Masuka's chair. In the next shot as Masuka leans down to pick it up it is closer to the bin. (00:39:30)
Other mistake: An extra is walking in the background with a measurement wheel while Debra is talking with Lundy outside of the warehouse. The character is hovering the wheel over the ground and not actually using it for its intended purpose. (00:17:10)
Visible crew/equipment: When they are at the crime scene, you can plainly see the camera and camera man reflected in Special Agent Lundy's sunglasses several times. When it is a close up shot, it is even more obvious.
Revealing mistake: When Deb is texting "Looks like it" to Anton, the buttons she presses on the keypad do not correlate with the message itself and the phone doesn't show any auto-correct feature being utilized, indicating that the text message was added in post production.
Dirty Harry - S4-E5
Visible crew/equipment: Before Debra is taken away in the ambulance Dexter is physically stopped from getting closer to the scene, and when Batista tells the cop that Dexter is one of them, in the next shot facing Dexter the reflection of the moving overhead boom mic can be seen on the car window. (00:04:05)
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.