Revealing mistake: In several supposedly outdoors scenes in the camp, in particular, when Frank is welcoming Margaret back, there's a very obvious indoorsy echo. (00:22:10)
Continuity mistake: In the shot where Hawkeye presents the Spam lamb to Henry, he sets it down, and Henry asks him what it is. Behind Henry, you can see Hawkeye's shadow - and see his lips mouth the word "A Spam lamb". The camera then changes to a close-up of Hawkeye, saying "A Spam lamb".
Continuity mistake: After Klinger's file cabinet gets bombed, BJ appears in the window. He's facing directly ahead, head slightly tilted to the right. When the camera goes to a close up, he's in quarter profile, left cheek forward. In the long shot, he's back to the original position.
The Sniper - S2-E10
Revealing mistake: When The Sniper begins his attack, a sandlot football game is in progress in the compound. As the quarterback goes back to throw a pass, he waits just long enough for the ball to be "shot" out of his hand.
Plot hole: In a tape home, Winchester implores his father to get him out of the MASH and suggests that Senator Griswold should help as his father "paid good money for him". As the length of time the show ran so exceeds the length of the war itself, inconsistencies for dates are perhaps unavoidable, and it might not be fair to call them mistakes. In this case however, the writers/producers have carelessly picked a senator who only served from November 1952, making dating inescapable. As Charles, in the same episode, complains how hot it is and everyone is in shirtsleeves, it must be very late spring, perhaps May, at best, just a month before the end of the war. To emphasize the mistake, by the next episode it is late fall/early winter. (00:12:45)
Requiem for a Lightweight - S1-E3
Audio problem: When Margie is saying she is impressed, Trapper replies "I should hope so", but his mouth doesn't move.
Revealing mistake: A number of times, during the night periods (for instance, during an OR scene) the windows are dark when the lights are on in the room. Then when the lights go out, the windows brighten up. They, and the interior lights seem to alternate lightening and darkening.
Revealing mistake: In the first scene, when the camera pans back, in the upper left corner of the screen you can see that a part of the tent has been removed to make room for the camera. (00:02:15)
Continuity mistake: After Hawkeye and Trapper cut the bomb wires, Trapper kneels down to see if it's still ticking, and his hand disappears from his helmet. (00:23:05)
Factual error: In later episodes, in the officers' club, there is a Huey helicopter hanging from the ceiling. Hueys were not put into service till three years after the war ended.
Continuity mistake: Near the end, Margaret and her husband are shown leaving for their honeymoon in a jeep. You can see Trapper and Hawkeye in the background with a wheelchair. Then it cuts to an emergency room surgery session that appears to be a throw-in scene showing them worn out. Then, after the commercial, it shows Margaret leaving again and Trapper and Hawkeye with the wheelchair in a full scene.
Preventative Medicine - S7-E22
Continuity mistake: In Potter's quarters Klinger's arm holding the chicken goes from in front of his chest to by his waist between shots.
Visible crew/equipment: As Col. Whiteman leave Sentry Post 1, where Klinger is, the boom mic is reflected on his helmet.
Continuity mistake: In Henry's office, the white cloth Radar has on his ears changes positions from shot to shot.
Continuity mistake: In the Swamp, Pak pulls out a stack of money, holding it by the end. In the closeup, he is holding it by the middle.
Continuity mistake: When in the Swamp, and they show Hawkeye numerous times, the door to the stove is open, then closed, then open, etc.
Visible crew/equipment: As the camera pans around the bed in Margaret's dream, you can see the camera man and equipment reflected. Also, when the soldiers march past, the shadow of the microphone rig plays across each one as they pass the bed.
Dear Sigmund - S5-E7
Visible crew/equipment: As Potter is panning with the binoculars, you can see the lights, camera and crew reflected in the lens.
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