No spoiler alert here—the details are all nonspecific.
George R.R. Martin, author of the Song of Ice and Fire series and the seeming victim of a recent crisis of faith, has an announcement that can vaguely be called “news.” Speaking with IGN, he revealed that there’s a “twist” that take place in the novels, but that is now impossible in Game of Thrones because of certain life-and-death plot diversions the HBO writers took from Martin’s text.
“It involves a couple of characters, one of whom is dead on the show but not dead in the books,” Martin said, in the course of the 25-minute interview. “The show can’t do this stuff, because they killed a character that I have not killed.”
As both Martin and Vulture both pointed out, there are at least 15 characters that fit that description. If you want to speculate, or watch others do it, check out the Reddit thread where this is being widely discussed. In the meantime, let me echo the top comment from that page: We’d love to read it, Mr. Martin.