Continuity mistake: In the first episodes, Col. Blake's wife is named Mildred, later her name is changed to Lorraine. (In case of confusion, when Col. Potter came to the 4077th, his wife was also named Mildred, but that's not relevant).
Continuity mistake: In the first season of M*A*S*H, Hawkeye mentions that his father lives in Vermont, in the later seasons his father lives in Crab Apple Cove, Maine and has never left there in 30 years.
Continuity mistake: At one time, Col. Potter talks about how his daughter is an only child. In a different episode, he mentions that his son's wife is having a baby.
Continuity mistake: There are two sets of the 4077 compound - the outside Fox Ranch set located at Malibu Creek State Park, and the second set inside the studio's Stage 9. The layout of both sets are quite different - Studio 9's set was vastly smaller than the Ranch set, so things such as the proximity of the Swamp to the mess tent are different between the shots filmed at the Ranch and shots filmed at Studio 9, when seen within the very same episodes.
Continuity mistake: In an early episode Hawkeye receives a sweater knitted by his sister. Subsequently in the show he has always been an only child.
Continuity mistake: In the episode "Change of Command", Potter asks Mulcahy if he "can do Methodist". In the episode "Period of Adjustment", Potter asks Mulcahy "You wouldn't lie to a Presbyterian, would you?" Just what denomination *is* Potter?
Factual error: In one episode in the hospital tent, Hawkeye hands a Hershey Chocolate Bar to a wounded soldier lying in bed. The back of the candy bar had a Universal Product Code (UPC) on it. UPCs were not used until the 1970s.
Continuity mistake: In the pilot episode it says the year is 1950. In the episode "Radar's Report" in season 2 Radar says it's 1951, yet in the episode "A War for All Seasons" in season 9 we are told at the beginning of the episode they are going from 1950 to 1951.
Deliberate mistake: These errors I assume were caused by the show running 3 times longer than the real war. Col. Potter arrived at MASH in September 1952 a year before the war ended, then a few months passed by Margaret got engaged for 8 months, then Frank Burns left and Charles took his place. In this time the war would have ended. In the final episode Margaret mentions Charles has been there for 2 years. Which means he would have arrived in 1951 a year before Trapper and Henry left and Col. Potter arrived. It's a lot to take in.
Character mistake: When Colonel Blake's wife is having a baby, he is on the phone with the hospital as tells them to tell 'Mildred' he loves her and will talk to her soon. His wife's name was Lorraine.
Continuity mistake: In the episode "It Happened One Night", Klinger has B Positive blood type. In the episode "C*A*V*E*", Klinger has AB Negative blood type. Blood types *never* change.
Other mistake: Across the show, several times doctors looking at an x-ray picture say things like, "Looks like he swallowed a whole junkyard" or "His belly is full of shrapnel." In fact, shrapnel, being metal fragments, should show up as clear white spots on the x-ray pictures, but none of that is visible in any of the x-rays shown throughout the show.
Factual error: Throughout the series, actors are seen wearing cuffless permapress fatigues. Neither cuffless nor permapress utilities were available until the 1970s.
Continuity mistake: In any scenes in the CO's office, look at the windows - they're sometimes opaque, sometimes clear. Also the windows in the OR. Usually semi-opaque, once, they were totally opaque, and made of a thick plastic. This was because it was very cold outside, and Hawkeye used a scalpel to cut them open to let in cooler air.
Factual error: Throughout the series, there are repeated instances of "I Corps" being pronounced, "Eye Corps." Since Army corps are designated by Roman numerals, it should be pronounced, "First Corps." (I wonder how they would have pronounced "III Corps" or "XVII Corps?").
Continuity mistake: In early episodes, Margaret's father was dead. Later, he was alive and visited the camp. Also, in a early episode, Hawkeye writing to his father, tells him to send his love to his mother. Later, he tells Winchester his mother died when he was a child.
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.
Factual error: Nearly every show depicts the use of disposable plastic syringes, which weren't introduced till 1955, two years after the war ended.
Continuity mistake: In one episode, Frank's middle initial is M, then later it's D. His full name was Franklin Delano Marion Burns.
Factual error: The Spot-A-Card pinball machine in the Officers' Club didn't come out until the 60's.
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