Factual error: When the can of beans explodes all over Hawkeye and Hotlips, they act as though the beans are cold. In fact the beans would have been at boiling temperature as the can overheated, causing it to explode, and the two would have been burned.
Revealing mistake: When Hawkeye and Margaret turn toward the stove, and he says "Are you hungry?", watch the can. Though it is meant to look like it blew the top off, it is plain that the top was just set into the top, and a blast of air was used to blow it open. This is because of the sound, and a stream of vapor that often accompanies a blast from an air gun. A sealed can set on a heat source would have burst not only from the top, but also from the side at a seam, or the bottom. Also, they were about five feet away from the can, and the beans went up, not out at a ninety degree angle. So they wouldn't get beaned. Beans on the face were for comic effect.
The Late Captain Pierce - S4-E4
Other mistake: When Digger shows Hawkeye the copy of the death certificate, Hawkeye steps out of the shower stall and its door swings back and forth. As Hawkeye wipes his face with the towel, we can see the top of the flesh colored covering Hawkeye is wearing. (00:08:05)
Revealing mistake: When Radar carries the field phone from Col. Potter's desk over to Hawkeye who is squatting in the corner, the phone doesn't have a phone line attached. It is an EE-8 Military Field Phone, which is a wired device, not a radio - even if Radar elsewhere says he has somebody "on the radio" when in fact he is using an EE-8. (00:17:35)
Other mistake: As Burns, driving the tank, runs over the Swamp, there's no sign of the still anywhere.
Other mistake: When the Colonel Griswald of the 1st Cavalry goes to leave camp, the Jeep he gets in is marked "4077 MASH" on the bumper instead of 1st Cav.
Continuity mistake: Before The Sniper starts firing, Hawkeye is carrying the two bottles of scotch firmly gripped by the necks. A second later, he's barely holding them by the caps, in preparation to drop them.
Continuity mistake: The "KILROY" Hawkeye writes on the window as BJ looks out was in the center with his fingertip. Later, as Hawkeye looks out, the "KILROY" is now closer to the frame, and the lettering is different, a small capital A in front of it, and lines above and below it.
Dear Mildred - S4-E7
Revealing mistake: In the shot where the horse kicks in the door, there's actually two shots - a kick, a quick cut, another kick. If you look closely, you can see how the hoof of one of the horse-leg props breaks off during the first kick. The second kick is performed by one leg only. If you look really closely - and probably use the slow-motion function - you can even make out the hand of the one wielding the leg through the hole made in the first kick. (00:18:10)
Dear Mildred - S4-E7
Continuity mistake: The injured horse that Radar, BJ, and Hawkeye go after, and who's later gifted to Colonel Potter, is played by different horses.
Dear Mildred - S4-E7
Visible crew/equipment: In the scene where BJ, Hawkeye, and Radar are in the motorcade talking about the horse, Radar is talking about how he doesn't want to give away the horse. He is standing in front of some barrels and when he moves, you can see some of the crew, the horse owner, and what looks like another horse in the background for 1 second.
Dear Mildred - S4-E7
Revealing mistake: When the camera is in a close up on Dan the pilot, the sound is a chopper that is running full power, but you can tell that the main rotors are turning slowly to simulate flight.
Dear Mildred - S4-E7
Continuity mistake: When Colonel Potter's in his office finishing up his letter to Mildred, right before Margaret, Frank, and the others walk in, the things on his desk change their positions between shots - such as the paper tray moving from Potter's left to the right side of his desk.
Dear Mildred - S4-E7
Continuity mistake: In the episode when Radar gives Potter the horse, they keep referring to the horse as "he". In later episodes, the horse is a mare named Sophie.
Continuity mistake: When Margaret pins on Frank's purple heart, his scrubs are very damp with sweat. When he enters the swamp, they're practically dry. (00:03:50)
Plot hole: When the truck with the orphans arrives, Col. Potter and Nurse Cratty greet each other like one would an old friend. Being new to Korea, Col. Potter doesn't know Nurse Cratty - as becomes clear from his reaction to Hawkeye's explanation to BJ earlier (who doesn't know Nurse Cratty for the same reason, but that's not relevant for this mistake). Note that in other parts of the series Col. Potter is depicted mostly as somewhat old-fashioned and reserved towards strangers. (00:05:20)
Continuity mistake: When Hawkeye is examining the orphan who is wiggling his tongue, there are two camera angles: one of Hawkeye from the front and one of the orphan over Hawkeye's shoulder. In the first shot, the tongue depresser that Hawkeye is holding is horizontal, in the other it is vertical. This flips back and forth a couple of times. (00:06:30)
Dear Peggy - S4-E10
Character mistake: When he sees the 'P' for Protestant on Davis' chart, Col. Hollister states "A lot of your Davises are 'J's, you know" implying that Davis is a Jewish name. In fact, the ID tag code for Jewish faith was 'H' for Hebrew. (00:18:45)
Dear Peggy - S4-E10
Audio problem: As Hawkeye calls for X-rays, you can see Klinger hollering a reply through the door, but nothing is heard. (00:21:30)
Continuity mistake: After Hawkeye and Burns drive through the puddle, the mud on the colonel's face is different in the long and closeup shots. (00:02:00)
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