Other mistake: When Carter goes to switch the full cup for the empty one, he accidentally sloshes water on Newkirk's hand and the fan of cards in it. When Carter goes to move the cups, look at Newkirk's hand. The cup exchange happens in front of the hand. The hand is out of range of the water.
Audio problem: When Hogan is taking pictures of Shultz in front of the halt-track (really, getting shots of the control panel), in the second photo, the shot is shown as if it's Hogan looking through the camera. He starts on Shultz's face, but pans down and to the right to get the control panel. Just as the view gets on the panel, you hear the shutter click, just before the camera fully settles on the panel.
Revealing mistake: When the guys are by the half-track keeping Shultz busy, if you look just above and to the left of Shultz's head, you see some tall fencing at the top of the hill in the distance. From the set up, if appears to be a baseball field outside of the studio property.
Revealing mistake: When the men are in the yard during their exercise period playing football, there is a wide shot of the yard. The half-track is to the left of the screen with Shultz next to it. If you look above the barracks, the Desilu water tower and a large metal building, possibly a studio building, can be seen.
Other mistake: In the end of the episode, after LeBeau said "Hit the deck!", the camera looks to the half-track. According to the plan, there was to be a nitro charge inside it to blow it up. Watch the tarp toward the front. It twitches upward in response to a small explosion on the ground, rather than inside. The sound effects complete the illusion.
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