Revealing mistake: When Schultz is on the way out after giving the guys the news about the inspections and roll calls, you can plainly see that the background behind him is a matte painting. This is easy to spot because the building in the distance is lighter in color, and there are ripples in the fabric used.
Factual error: When the farmer sticks his pitchfork into the haystack, a scream is heard, but then everybody emerges unscathed. Wounds from pitchforks aren't like sitting on a brass tack - even a quick jab usually earns you a trip to the surgeon (I've seen such wounds), a vigorous stab like the one in the scene would go through limbs and could easily kill a man. There's no way anybody would walk away from such a hit.
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