A savvy Redditor, _honeybird, went to the Cushing Library and looked at the original manuscript for A Dance With Dragons by George R.R. Martin, including editorial notes and Martin's responses. And... it looks like Martin shot down a major fan theory and revealed a huge spoiler.
Top image: The Shield That Guards the Realms of Men by Rene Aigner, via Westeros.org
Seriously, major spoilers ahead!
For years now, fans have debated the identity of Coldhands, the mysterious guy who meets up with Bran Stark. One popular theory is that Coldhands is Benjen Stark, Bran's long-missing uncle. But responding to an editorial query, Martin wrote a brisk "NO" to that idea. See for yourself:
This, of course, will mean nothing to fans of the TV series who haven't read the books, given that Coldhands is apparently MIA on television. But it's a big deal to book readers. We've reached out to Martin through his publicist, and if we get any clarification we'll add it here.
Also, there were a few "missing" chapters from Dance With Dragons, which fans believe have been held back for inclusion in the next volume, Winds of Winter. But _honeybird was unable to find those, and it looks like those "missing" chapters were not included in the manuscript sent to the publisher, rather than that they were later removed.
[Update: Wars and Politics points out that one of the chapters that was left out appears to be a Bran chapter, which would have taken place after the final Jon chapter in the book. This chapter was pushed back to Winds of Winter, and may contain some pretty huge reveals about events near the Wall.]
Also, Martin's editor (Anne Groell, I'm guessing) is as tired of some of Martin's overused phrases as many readers are — she tried to make him cut down on the use of "Soon or late," "Words are wind," and endless mentions of Davos Seaworth's lost fingerbones, at the bottom of the Blackwater. She did succeed in making him give more context in an early Jaime Lannister chapter, and resolved some inconsistency in whether Reek still had any teeth left. Plus the order of some of the later chapters got swapped around.