When someone gets resurrected from the dead
This is a tricky one. On science-fiction and fantasy shows, resurrection is a common trope that can be used to great effect. Buffy the Vampire Slayer is one of the best shows ever made, and Buffy died twice. But in Buffy's case, each death wasn't just a cheap storytelling decision -- it served a greater narrative purpose, and the consequences of her coming back were explored in depth. I hope Jon Snow's inevitable return on Game of Thrones will be treated with similar gravitas.
Pregnancies, weddings, interventions, infidelity, or having a popular character clone himself are all reliable ways to goose the ratings, but death is the ultimate watercooler moment. So when creators bring a character back from the dead, like when Locke died and then got resurrected on Lost, it can start to feel like there are no real stakes to the drama. Of course, Locke's body was actually being controlled by the Smoke Monster during the time I'm thinking about, but, uh, that's another problem altogether.