Dumbledore is the headmaster of the school in the Harry Potter series.
Towards the end of the series, a bunch of people found out a spoiler and spread it around like wildfire. The victims were understandably not happy.
I actually had the end of King Kong ruined for me by this Penny Arcade comic, which was making a point about spoilers and movies:
I've also had a lot of stuff ruined for me that was more recent, like Game of Thrones and season 2 of Sherlock. The arguments were that the books have been out even though the TV series hadn't gotten out to that point yet, and that the stuff was already out in England even though it hadn't been released in America yet, respectively. Neither was comforting.
The way I see it, if you haven't seen it, it's still new to you. I've actually avoided making Star Wars references involving Vader and Luke just because I didn't want to take the chance, although admittedly it's unlikely that people have been able to avoid that spoiler for this long.
Your own opinions may vary.