Peace on Us - S7-E2
Revealing mistake: When Colonel Potter's in the Swamp telling Hawkeye, BJ, and Charles that surgeons' rotation points have increased from 36 to 45 Hawkeye becomes incensed, and in these two shots from this angle we can see the edge of the Swamp set at the top, left corner of the screen.
Peace on Us - S7-E2
Revealing mistake: Klinger says "The sun is going to ruin my comb-out", fingering his hair net. He has curlers under his hair net, but his hair is not around the curlers, they're just stuffed into his hair net. (00:01:10)
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