1. "Abyssinia, Henry" (M*A*S*H) - The end of the third season of M*A*S*H saw Lt. Col. Henry Blake (McLean Stevenson) getting discharged and sent home. After saying goodbye to the people at the 4077th, Henry gets on a helicopter and leaves the camp. Later, in the final scene, Radar goes into the O.R. and informs the gang that Henry has died when his plane was shot down over the Sea of Japan. This also turned out to be Trapper John's last episode. However, it wasn't known at the time.

2. "The Person Formerly Known as Lowell" (Wings) - Thomas Haden Church left Wings after the sixth season for his own sitcom "Ned & Stacey". Early in the seventh season, he returned as a guest star to give Lowell Mather a proper send-off. In the episode, Lowell, returning from vacation, witnesses a murder and turns the guy in. Later, he discovers that the murderer was a mob boss. Lowell gets threatened and joins the witness protection program. After a tearful farewell with the gang at the airport, Lowell leaves Nantucket forever.

3. "Hello, I Must Be Going" (Ellen) - Arye Gross, the last original member (besides Ellen, of course) of the sitcom has his final episode early in the third season. His character, Adam Green, gets a job offer in London and prepares to move away. Before leaving, Adam confesses that he loves Ellen. He returns when his flight has been delayed.

I won't list them all, so others can join in. Feel free to add to the list.

"I Do and Adieu"- Diane's last episode on Cheers (aside from the finale). I thought it was a great way to send off the character and end the Sam/Diane relationship. The way that, even though she promised she'd return, Sam just knew that she wouldn't. And I love the final scene of the two of them dancing. I think it worked out well for the show. They had 5 years of Sam and Diane and I really never could see the characters actually getting married.