The ruining of a surprise in a book, movie, tv program, etc.
So, SasukepwnsItachi only to discover that Itachi was a good guy and that he massacred his clan on orders from the higher ups. Turns out the entire UchihaClan was planning to take over the village. Ironic isn't it? Now Sasuke wants to crush the village.
Oh, and Snape is going to kill Dumbledore But he actually is good. And Harry will live.
Oops, I forgot to put a Spoiler Warning didn't I? Well, hope the Spoilers didn't ruin your day.