Revealing mistake: In the close ups of Maj. Zolle, it can be seen that the spectacles that he has on have no lenses.
Continuity mistake: The emblem on the left side of Schultz's helmet is mostly gone. Later when apprehending Stoffel, the emblem is complete.
Visible crew/equipment: As Hogan and Le Beau crawl through the car and shut the door, the camera rig, equipment, and cameraman are reflected in the window.
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