So far Game of Thrones hasn't been shy about changing things from the A Song of Ice and Fire novels to make the adaptation to HBO work better, but it's almost always been either minor things or things that worked better as streamlined. That's all going to change next year, because behind-the-scenes images are revealing that season five is taking huge, major deviations from the source materials.
Spoilers for A Feast For Crows and A Dance With Dragons follow.
The biggest change is video that reveals Jaime Lannister hanging out in Dorne, Westeros' version of Spain, with the family of the slain Red Viper. That's a huge deal because he hasn't been to Dorne at all in the books, and he's been actually spending all of the last two novels fucking around in the Riverlands, not really being that interesting (as I recall). Jamie going south to Dorne makes sense - his daughter is being held there, and I could see him allying with the Martells - but it's a major spoiler if it's something George RR Martin is planning to do in The Winds of Winter.
Meanwhile other behind-the-scenes shots have revealed Tyrion is also getting ahead of his book-self in season five. In A Dance With Dragons Tyrion travels to Pentos and then tries to find Daenerys, having adventures along the way. He does not truly meet her in book five, although he gets close when she re-opens the gladiatorial pits for her wedding celebration. Photos from the Game of Thrones shoot in Spain show that Tyrion is actually at the wedding, and as a guest of Dany herself. Some have speculated this means his journey won't happen as it does in the books, but I suspect the wedding happens either in the finale or the penultimate episode, giving Tyrion plenty of time to go riverboating and get sold into slavery along the way.
I'm sure these changes are just the tip of the iceberg. We were wondering how the show would deal with the largely event-free last two books (which took place simultaneously), and the answer is 'By changing them up in big, big ways.'