Factual error: When the camera looks at Lt. Clark, her rank insignia is horizontal. It should set from the apex (point) of the stitching diagonally (top end toward her face).
Continuity mistake: As Charles talks to the guys in the Swamp, his right hand alternates from up at his chest to down by his side several times too quickly to be normal.
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