Slack Tide - S4-E7
Other mistake: When Dexter shreds Laura Moser's file, he actually shreds a file for Estrella Carazo. (00:32:57)
Dirty Harry - S4-E5
Other mistake: When Lundy is shot and falls to the ground, there is no visible injury even though he is facing the camera. Later, Masuka says he was shot twice, once in the chest and once in the neck. The shooter was facing him. We should have seen the blood.
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)
Other mistake: There's no way Arthur could have known Scott was 10 years old. In fact, it's highly likely he would not have known any of the boys he killed were 10 years old.
The Getaway - S4-E12
Other mistake: The tub Rita was found in was full of water, to the rim. Volume displacement dictates that when Arthur got out of the tub the water level would have dropped to well below the tub's rim.
Answer: No, Harry is not the real father. While he did have an affair with Dexter's biological mother (Laura Moser), this was when she was his C.I. Dexter was already born at that point.