Comrades in Arms: Part 2 - S6-E13
Other mistake: During the surgery, after Hawkeye and Margaret reach the 8063rd, Margaret is explaining the procedure. She says that they are using a new vascular clamp designed at the 4077th. It appears that this is the clamp designed and built three episodes later in episode 16 "Patent 4077."
Other mistake: In the episode where Radar is talking to Corporal Hendrix, Radar says that he was thinking about getting a tattoo himself. But in season 3 episode 7 when Col. Blake was giving Radar his physical he had an anchor on his right arm which he said was his new tattoo. (00:08:20)
Other mistake: When Penobscot is approaching the group, you can make out "4077 USA MASH" on the bumper, even though the the numbers and MASH are partially obscured.
Answer: Even an Army field hospital would have SOME weapons for self-defense if the enemy attacked. What's meant is that it's not a combat outfit, and therefore wouldn't have enough weapons to actually attack anyone. (It's also mentioned a few times that the Officer of the Watch is supposed to carry a pistol at all times, but Hawkeye refuses to do it when he's in charge and Colonels Blake and Potter don't enforce the rule).
Captain Defenestrator