Factual error: When Radar, Hawkeye, and BJ find the letter from cpl. Benson to I Corps, Radar says that cpl. Benson's been here since June. Later in the episode when col. Potter, Radar, Hawkeye and BJ confront Benson, Benson says he is there on orders of a colonel who did not get treated quickly enough. Potter says he remembers this colonel was there a couple of months back. BUT, this episode of M*A*S*H takes place during Kentucky Derby Day. The Kentucky Derby has always been held the first Saturday in May. So, if Benson has only been at the 4077th for two months, and he arrived in June, then this episode should be taking place in August, not May.
Visible crew/equipment: While in the mess tent, after Margaret's grossed out by the smell and walks away we hear Charles start playing again, and when Colonel Potter says, "Just make sure the three of you work this out fast," in this shot we can see the actor's T-mark on the floor between Potter and Hawkeye, where Margaret stood a moment ago.
Continuity mistake: In the episode 'Chief Surgeon Who' (season one), near the end of the episode we see Radar in Col. Blake's office smoking a cigar with ease, but in the first episode of season six when Col. Potter offers Radar a cigar, he doesn't know how to smoke it and he throws up after he puffs it.
Visible crew/equipment: After Potter tells Klinger that his lawyer, Schaeffer, is a fake who's impersonated a chaplain among other things, when Schaeffer tells them that 25 couples are living in sin, in the next shot two curved chalk actor's marks can be seen on the floor. (00:35:45)
Continuity mistake: While Colonel Potter is painting the portrait of Charles, the portable military field desks behind Potter are all open - note the front doors are flipped down, but in the closeup of Charles holding the painting of himself the doors are folded up. And the books, etc. atop the field desks change as well.
Continuity mistake: When Colonel Potter, BJ, Hawkeye, and Charles are in the changing room, Charles wants to be alone so he takes his tan jacket off the first nail on the right and leaves, but in BJ and Hawkeye's closeups the tan jacket is still hanging on that first nail, then it vanishes again.
Visible crew/equipment: When Colonel Potter surprises everybody with his "little Olympics" idea in the mess tent, after Margaret comments, "I just know I'm not going to have anything left for Donald," at the start of the next shot the boom mic casts a very quick moving shadow on BJ.
Plot hole: When Hawkeye and BJ buy Margaret a new ring, it takes a typo in the inscription to give them away - which makes both of them and Mr. Shin almost clairvoyant. Not only did they somehow guess Margaret's ring size, but Mr Shin - phew, Houdini and Uri Geller together don't hold a candle to this guy - actually somehow divined the exact type and font of the inscription in the original so perfectly that Margaret actually only spotted the difference from the missing 'n.'
Continuity mistake: When Cpl. Hendrix is examined on the operating table, Klinger and Hawkeye admire his tattoos. Klinger asks "do you really think the real Louise is built like that?" and Hawkeye replies, "If she is, I'm in love". Later, when Radar talks to Hendrix, Radar says, "Her face is still pretty, and her legs are the greatest." Hendrix asks, "What about the rest of her?" and Radar replies, "There isn't any." How could Hawkeye and Klinger admire Louise's build if the tattoo was that shot up? (00:01:10 - 00:07:20)
Other mistake: During the surgery, after Hawkeye and Margaret reach the 8063rd, Margaret is explaining the procedure. She says that they are using a new vascular clamp designed at the 4077th. It appears that this is the clamp designed and built three episodes later in episode 16 "Patent 4077."
Factual error: Charles mistakenly injects a Post Op patient with curare instead of morphine. This would have been hard to do. Curare was not approved for use in Korea by the U.S. Army and it would not have been there. Even if it had been, curare was used in conjunction with anesthetics in the operating theatre. It would make no sense to have it in the Post Op. (source pg. 14 "Notable Names in Anasthesia" by J. Roger Maltby, Royal Society of Medicine - Great Britain). (00:12:20)
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)