According to the Morning Call, HBO's president of programming, Casey Bloys, announced the network's extensive plan to keep the series finale spoiler-free: shooting multiple endings to keep the cast, crew, and, most importantly, fans in the dark.
During a speech at Moravian College in Bethlehem, Pennsylvania, Bloys said:
[For the ending of Game of Thrones] they’re going to shoot multiple versions so that nobody really know what happens. You have to do that on a long show. Because when you’re shooting something, people know. So they’re going to shoot multiple versions so that there’s no real definitive answer until the end.