Continuity mistake: As Klinger and Margaret are talking about the date, he takes a sip of his drink and goes to put his hand down. After the angle changes and he goes to say his line, he repeats putting the glass down.
Continuity mistake: Margaret comes in to the Officer's Club, and tells Klinger that the drinks are on her. He puts down two glasses, and starts to pour. When the camera cuts to a close up, he's just starting to pour into the same glass again.
Answer: The C-42 was a military variant of the Douglas DC-2. Very few C-42's were built, so it's questionable that Potter would specifically have seen that particular model, but, given his military background, it's not entirely unreasonable that he might use the military designation even when the aircraft in question is actually a civilian DC-2.
Tailkinker ★