As soon as Michael left the Office, that was the end.

Carrell was such a linchpin for the show’s ensemble that the writers had literally no idea what to do to replace him. So they simply assassinated Andy and made him an incompetent idiot, despite his well-educated character being the total opposite. The show doesn’t recover in those last two seasons, though the finale wrap-up is pretty good.

Another one that comes to mind is Buffy dying in S5.

The beginning of S6 is about all the good that comes from it. Then everything nosedives, character personalities are completely decimated, plot lines make way less sense than they ever did, and the villains become so much less interesting. You don’t care because what are they gonna do, kill her again? The Potentials being terribly uninteresting in S7 is just the icing on the cake.