LeBeau and the Little Old Lady - S3-E25
Audio problem: After Hogan and company get into the car to go to town to get Lebeau, Klink is walking around to get in the passenger side, Schultz takes off, leaving Klink standing alone. After Klink calls out, you hear the screeching of tires sliding on pavement. Trouble is, the camp doesn't have any pavement.
Sergeant Schultz Meets Mata Hari - S3-E4
Audio problem: You hear Carter ask, "Hey, what are you doing with my mattress?" as the Gestapo searches the barracks, but his mouth doesn't move.
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