1. Lisa Bonet, "A Different World" - The show wasoriginally a spin-off of "The Cosby Show" featuring Lisa's charactergoing off to college. But then she and Lenny Kravitz were having a baby in real life. She left and eventually returned to "The Cosby Show" after having the baby.
2. Paul Schneider, "Parks And Recreation" - When he left after season two, the show brought on both Rob Lowe and Adam Scott.
3. Craig Kilborn, "The Daily Show" - It's hard to believe there was ever a host other than Jon Stewart, but Kilborn had it from 1996 to 1999.
4. Shelly Long, "Cheers" - It was Shelly's choice to leave the show, and it turned out to be a bad choice for HER. But replacing her with Kirstie Alley and ending the Sam and Diane romance angle probably saved the show.
5. George Clooney, "ER" - Clooney was the breakout star but his departure in 1999 didn't kill the show. In fact, "ER" survived a million other cast shake ups before it ended, without ever really losing what made it great.
6. Katherine Heigl, "Grey's Anatomy" - By the time Heigl left the show, her character had gotten pretty annoying ... sort of like the public's real life perception of her. No one was really sad to see her go.