Wayne Rogers decided to leave the show because he felt that Trapper John had become more of a sidekick to Alan Alda's Hawkeye than the equals they were supposed to be. 20th Century-Fox sued Rogers, but its case collapsed when it transpired that he'd never signed his contract. The reason Rogers cited for this was an archaic "morals clause", which he wouldn't accept unless the studio signed one for him in turn.
The trivia items below may give away important plot points.
After the news of Col. Blake's death shocked the world, the very next night on The Carol Burnett Show (1967), the opening shot was of "Henry Blake" in a smoking raft, waving his arms, hollering, "I'm OK!" I'm OK!"
In the third season finale "Abyssinia, Henry", Henry Blake is sent home, to coincide with McLean Stevenson's departure from the show. In a surprise twist at the end of the episode, the characters learn that Blake's plane was shot down en route to Japan, and everyone aboard died. This was kept a surprise from the cast until the rest of the episode had been filmed. The cast members, except for Alan Alda, did not see the final page of the script, with Blake's death, until they were on the set shortly before the scene was shot. A technical mistake required a second take to be filmed, and in that take one of the cast dropped a surgical instrument on the floor, banging loudly as it hit. This mistake was left in, since it so well fit the scene. McLean Stevenson was on the set as the scene was shot, and left almost immediately afterward. A catered wrap party had been prepared, but after filming was finished, nobody felt like celebrating, and they simply went home.
When Henry Blake died in the end of "Abyssina Henry", the show's writers and producers were bombarded with hate mail and letters of protest. Some people wrote letters saying that Henry Blake is alive and well in their city. The point the producers and writers were trying make was that all soldiers, when they were discharged did not necessarily return home safely, when their tour of duty was over.
When Radar enters the operating room, actor Gary Burghoff was the only actor who knew the real lines concerning the tragedy of Henry Blake. The other actors thought Radar was going to deliver a wire from Henry on his safe return. The director told everyone to stay in character after Gary said his lines. When the camera pans to the others, their reaction is genuine.