Factual error: As the staff car comes up the street, there are a number of neon signs lit up, as well as store fronts. This is wrong as during the night in wartime Europe, such lighting would be dark to avoid night bombing.
Continuity mistake: As Hogan's telling Klink about the drawings, his left arm is extended. The next shot his arm is resting on the cigar box, then back to being extended and notice the pencil under the note pad. Next shot, arm is back on the box and pencil is gone. (00:04:45)
Other mistake: When the boys are working on the "jigsaw" map, Klink and the guard come in. There is a gust of wind from their entrance. Trouble is, the gust that blows the pieces comes from behind LeBeau, blowing the pieces toward Newkirk's bunk, rather than toward the camera.
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