Continuity mistake: When Jack delivers the patient to the MASH and when he is taking photos of the operation, there is no wound on his right hand. It only appears when Col.Potter says that he treated it "months ago". (00:04:30)
Other mistake: Frank Burns states when he is talking to a patient who just got a purple heart, that he would like to get a purple heart, but in the earlier episode ‘The Kids', Frank Burns received a purple heart due to a injury, and in the S1 episode ‘Sometimes You Hear the Bullet', Frank Burns gets another Purple Heart. Despite them both being stolen, they still count on his army record.
Continuity mistake: After Jack wins a drink from Hawkeye and BJ, Hawkeye grabs the orange to inspect it for tears to prove his magnet theory. After the angle change, he grabs it again.
Visible crew/equipment: When the camera angle changes to look at the cover being put over the soldier's head, the camera is briefly reflected until the cover is locked in place.
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