Revealing mistake: Potter is taking the driving part of the test, with Rizzo riding along. In the long shot, as well as looking at Klinger stumbling along, you can see the Jeep occupants are stunt doubles.
Continuity mistake: In the Swamp, as Hawkeye asks about Peg's cookies, BJ folds the cards in his hand, and places his left hand over the top. In the close up, as he answers, he suddenly has the cards back in a fan.
Continuity mistake: As Rizzo turns to Potter to give the test score, he puts his cigar in his mouth, and his right hand on his papers. The next shot the cigar is suddenly in his right hand.
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 ★