Revealing mistake: When Col. Potter is in the helicopter looking for a new site for the camp, you can see landscape reflecting in the canopy. This never changes position although they're meant to be flying around. (00:22:30)
Revealing mistake: In the mess, you can see everyone has their food on metal trays and ceramic dishes. Sgt Zale, though, has his lemon pie in a polystyrene dish - so that it doesn't hurt Klinger when he gets it in the face. (00:08:15)
Revealing mistake: When Radar salutes Major Burns dropping the package, you can see wet spots on his uniform where he held the package. Before the bottle inside breaks, it is supposed to be dry. The crew probably re-used a single package for several takes of the shot. (00:04:45)
Revealing mistake: Klinger makes a big show of removing a big nut from the Jeep's leaf spring. The nut is clean and shiny whereas the rest of the undercarriage is painted and dirty. If you look closely, there are no screw threads where he takes it off either, there's another hex under it. The simple reason for this is that that nut doesn't belong there and never was affixed, Jamie Farr just pretended it was.
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