Two Nazis for the Price of One - S3-E17
Visible crew/equipment: A string is seen attached to Colonel Klink's cap that is used to yank off the cap after Colonel Mannheim fires his pistol at Klink.
Some of Their Planes Are Missing - S3-E2
Visible crew/equipment: When LeBeau, Carter and Schultz enter the visiting Germans' quarters, when the door is open you can see the sound stage floorboards are uneven in front of the door to the neighbouring barracks.
Visible crew/equipment: After Hogan goes down into the tunnel in Klink's quarters, the stove moves back in place. As it does, the wire on the floor pulling the stove back in place is visible.
How to Win Friends and Influence Nazis - S3-E7
Visible crew/equipment: When Hogan gets up in the hotel room they're all in, the shadow of the boom mic plays on the wall he heads toward.
Visible crew/equipment: As Klink's car pulls up, and Klink comes around the front of it, the shadow of the camera is visible.
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