Biggest Continuity Goof
I know it's been discussed a lot but having seen it just now, a huge error was the 'A War For All Seasons' episode which starts with a New Year's Eve party at the end of 1950. As IMDB explains,
'The opening and ending scenes have Colonel Potter leading the 4077th in celebrating the end of the year 1950 and 1951 respectfully. In his initial appearance in Season 4's "Change of Command", Potter replaces Colonel Henry Blake as Commanding Officer, and it is announced that the date of his arrival was September 17, 1952. Therefore Colonel Potter would not have been at the 4077th on New Year's Eve 1950 nor 1951. Neither Maj. Winchester nor BJ Hunnicut should have been there either, given the time they arrived in relation to Col. Potter. This also means Radar, Captain McIntyre, and Maj. Burns should have been present as well.'
Plus, the Korean War started June 25, 1950. That means a lot of MASH events would have been compressed into 6 months. Like, say, Trapper and Hawkeye going behind enemy lines to pick up wounded American prisoners held by the Chinese, who weren't even in the war until November 1950.