TheBoost said:
One that I thought of was "Count of Monte Christo" and how SPOILER ALERT in the book it's not clear who the count is, or what characters know who he is or not. But in the movie, with actors and closeups of reactions it's harder to pull that off, so even though the most recent adaptation was marginally faithful to the book, in one fundamental way the narrative was <span style="text-decoration: underline;">completely </span>changed.
It may be because I read an abridged version, but I think this was pretty clear for the most part. I need to find myself an unabridged copy, I loved that book a lot.