Visible crew/equipment: As Burkhalter is talking to Strauss and Klink in front of the office door, you can see the shadow of the boom mic moving over each as they talk.
Other mistake: The men are usually all wearing boots. When Carter goes up the ladder, it's clear that he has high-top sneakers on.
Continuity mistake: The whole operation is supposed to be at night, but when the original seven approach the gate to surrender, it's late afternoon, and very light.
Continuity mistake: In the tunnel, Hogan lays out the plan. The arms of the man next to him goes from crossed to by his side between angles.
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