Corrected entry: In this episode, Potter and company are being introduced to white phosphorous that is starting to be used. But in Season 2, Episode 1, "Divided We Stand", as Henry and Hawkeye come out of the O.R. a wounded soldier is brought in on a Jeep with white phosphorous burns, and they knew what to do.
Correction: "New" to the doctors on the front lines and "new" to the doctors back at HQ could be two different things.
It might be old news to the 4077th but new to the Army in general. Without asking a real Army doctor, Instructional briefings like this aren't optional. They don't ask if you already know it. The point of the scene is NOT "How many times has the 4077th already done this?" The point is "Potter thinks he's too old and can't hack it anymore, so IF the Army has learned something new, HE wants to hear it." And also shows us "Potter is on edge about something. Maybe we should call Sidney Green."
You could be the world's top expert in White Phosphorus, but if you're in the Army, and they tell you "You're going to attend a lecture on White Phosphorus," That's called an order and you do it, Mr. White Phosphorus, whether you like it or not.
Correction: Captain Simmons said the new weapon is "white phosphorus rounds." Phosphorus before then was likely used as part of artillery shells.
Correction: I know you can't worry about MASH's timeline or you'll go insane, but six seasons pass between this episode and that one. The Army medics could have learned some new things about treating the injuries in whatever time passed. And again, Col. Potter thinks he's slipping in this episode, so he wants to hear every word, and that's the main point of the scene.
Correction: Even if they knew how to deal with it at the time, the information might not have been common knowledge. As WP came to be used more frequently, the Army would send instructors to field hospitals to make certain everyone was up on the latest technique for dealing with it. (Col. Potter was also not in the earlier episode you mention, and he wants to hear the information).
Captain Defenestrator
Understood, but Potter was there in Season 4, Episode 24 "Deluge" when a WP case was brought in.
Movie Nut
Remember that the main plot of this episode is that Col. Potter made a rookie mistake that almost cost a kid's life, and is fearing that he's too old to hack it as a doctor anymore. If the Army's learned something he doesn't know, he wants to know it.
Captain Defenestrator
The dialog explicitly states that the enemy "has started using something new", which is phosphorus rounds. If they had to deal with it before, it's logically not so new, ergo the mistake is at the very least plausible.
Doc ★