For those of you who haven't heard of Seth MacFarlane, he was the creator of Family Guy, a funny animated sitcom show that ran from 1999-2001. It's sort of like The Simpsons, but with a lot more crude and non-family friendly jokes.
Unfortunately, Seth was killed in the September 11 attacks on board American Airlines Flight 11. Since he did the voicework for half of the major characters, they had to cancel it after he died.
So let's say he gets drunk the night before or caught in traffic and misses the flight. How long could Family Guy have gone on if he had lived? IIRC, it was starting to get popular, so I can see 2 or 3 more good seasons, but it might suffer from the inevitable quality decline that comes from forcing a show to continue running past its lifespan.