Factual error: The German agent pretending to be an American pilot is wearing a one piece flying suit from the 1960's when he is questioned by Hogan and the British escapee in the barracks. It even has the US Air Force logo on the sleeve, which didn't come into use until 1947, after the war.
Revealing mistake: When Schultz checks the barbed wire to see if it is fixed now, he noticeably wiggles the vertical wire, but only barely touches the horizontal one - because it is rigged to come apart, and will again if he handles it too roughly.
Other mistake: When the boys are listening to Klink's office, the lid is on the speaker instead of being off.
Continuity mistake: When Newkirk brings his hat loaded with candy, he hands it to Hogan with the badge on the top on Hogan's left. When the camera angle changes, the hat has been reversed.
Revealing mistake: When the boys are working on the fence, when the camera is on Hogan, you can see a modern metal building in the distance.
Answer: Nimrod's actual identity was never revealed in the series. It was only known that he was a British intelligence agent. Nimrod was not Colonel Klink. Hogan had only implied it was him as a ruse to get Klink returned as camp commandant, not wanting him replaced by someone more competent who would impede the Heroes war activities. The term "nimrod" is also slang for a nerdy, doofus type of person, though it's unclear why that was his code name.
raywest ★
"Nimrod" is originally a king and hero mentioned in the Tanach and taken into the Bible and the Koran. His name is often used in the sense of "stalker," "hunter," and sometimes figuratively as "womanizer" as in "hunter of women." I've never seen it used to denote a nerdy person, and although I cannot disprove that connotation, I think given his role, the traditional meaning is more likely the intended one.
Doc ★
It's widespread enough that Wikipedia has an entire section on it: https://en.wikipedia.org/wiki/Nimrod#In_popular_culture