It happened with The Vampire Diaries last night. It's happened with Justified twice in the past few weeks. Co-stars or recurring guest stars on current shows book new pilots. It's what they do! Actors want bigger roles and steady jobs! This is totally fine. And then outlets like Deadine report the casting, because, again, it's what they do! And then some fans read the news and get upset that the current show has been "spoiled" by the knowledge of this other show's casting. And . . . honestly, I think everyone needs to take a deep breath and calm down.*
First of all, knowing that someone's role will be reduced or eliminated does not actually spoil the plot. (And casting notices like these almost always claim that the actor will be available to return to his old show as a guest star, regardless of whether that actually happens.) It doesn't necessarily mean the character will die. He might move away. It might be something else entirely. "XYZ character will die in a car crash" is a spoiler. "XYZ actor may be leaving the show" is not. (Note: That was not a real spoiler, about anyone. I know nothing!) Sure, it may make you watch the rest of the season a little differently, but any show worth watching should be giving you plenty of reasons to stay engaged other than the future of one particular character, anyway.
But more importantly, we need to get past this idea that anyone has a right or a reasonable expectation to participate in public discussions of TV (or books or movies) while remaining in a spoiler-free bubble for whatever that person's definition of "spoiler" happens to be. If you're going to be upset by seeing Deadline headlines on Twitter, don't follow people who will retweet Deadline headlines. More to the point, if you want to talk about a show without hearing absolutely anything about the future, don't talk about it on Twitter or large, general fansites. Email your friends. Start your own forum with strict rules. There are options.
And this goes doubly for things that have already aired. Personally, I try not to tweet spoilers for a day or so (or at least until the West Coast has seen an episode) as a courtesy, but I don't actually think that anyone owes anyone this. If you know you haven't seen this week's episode of your favorite show yet and you really care deeply about not hearing about it, just don't go on Twitter or wherever else you know people will be discussing it until you see it. The people you follow don't magically know your viewing schedule, and frankly, it's not their responsibility to care. And for God's sake, don't throw a fit if someone mentions a plot point that aired months or years ago. (I refuse to even call it a "spoiler" at that point.) He won the basketball game without turning into a wolf. 27 years ago. If you haven't seen it yet, you can't care that much.
* Note: I am in no way saying people shouldn't get upset about a character or actor they like leaving a show. That's perfectly fine. I'm just saying they shouldn't get mad at other fans for talking about it.