What's the statute of limitations on spoilers? Obviously if you're personally talking to someone who hates spoilers and has yet to experience the surprise or twist that you could spoil, you don't do so (unless maybe you're evil), but what about public disclosure? In an episode of The Big Bang Theory, Sheldon spoils a Harry Potter book for Leonard, who had not read it yet, and of course it's a big deal for Leonard, but what about for the viewers? If it's plausible that Leonard hadn't read the book yet, then it's possible it's true for some viewers, too, and the spoiler for Leonard would have been a spoiler for that part of the audience. Now, the book in question, The Half-Blood Prince, was first published in 2005, being 8 years before the TV episode, and the movie of the book was released in 2009, 4 years before. Is that enough time to say "if you haven't seen or read it yet, too bad"? Should there have been a spoiler warning at the start of the episode? There's probably a time beyond which you can't expect the whole world to keep the surprise for you, but how long is that, exactly?
Mokalus of Borg
PS - The episode would have been difficult to pull off without the actual spoiler, though.
PPS - Unless they made up a fake fictional franchise, which they don't generally do on that show.