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)
Dirty Harry - S4-E5
Revealing mistake: After the intro at the start, it cuts to Dexter passing the fire engine, and the next long shot with a firefighter lifting the yellow tarp is a flipped shot; note the backwards licence plate, sign, etc.
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.
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)
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.