Continuity mistake: When Trapper turns around to ask Klinger for the status about the movie's sound, a small patch of his hat becomes wet from the roof's leaking, but in the next shot, his whole hat is instantly soaked. (00:01:55)
Continuity mistake: During the scene which culminates in Frank declaring the camp dry the patches of water on his helmet change position. (00:05:50)
Continuity mistake: When Father Mulcahy's finishing up his Sunday service with only Klinger, who's wearing a fetching red hat, there are nine stacked books at the corner of the table, but when it cuts to Mulcahy's closeup there are now only six stacked books (which will soon be used for the 11:00 Protestant service when Trapper and Hawkeye will pray for booze).
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