Revealing mistake: When the mail arrives in the compound, in daylight, everyone has 3 shadows. (00:00:50)
Visible crew/equipment: After Hawkeye and Trapper tell Frank that they made up Pioneer Aviation to help him look foolish, when Hawkeye picks up Trapper's duffel bag, as the shot pulls back the chalk T-mark can be seen on the floor where Hawkeye had been standing.
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 ★