I think Planet of the Apes probably counts

Let me say at the outset that I have no problem with characters in stories having things dramatically revealed to them that we, the audience, already know about. Heck, one of my favourite lines from Doctor Who is a perfect example of this:

"My home planet is far away and long since gone. But its name lives on: Gallifrey."

Yeah, yeah, we knew that decades ago - but it's still awesomely played.

No, what I'm thinking about in this post are dramatic reveals - the sort which reveal something shocking to the audience - which are lost, either through adaptation or prequels. I have a couple of examples:

-Dernhelm. The sequence where Dernhelm, knight of Rohan, reveals himself to be Eowyn in disguise - and then kills the Witch-King - is one of my favourites in the book. Dernhelm is a minor character up to this point - he looks like nothing more than a deus ex machina to get Merry to the battle - but that reveal is amazing. In the movie, though? We know it's Eowyn in disguise as soon as we see 'him' - partly because they gave her a completely non-concealing helmet, and partly because Merry immediately calls her 'my lady'. There's no reveal to us, and I miss it. It's a cool scene.

-Also from LotR: finding out what happened to Frodo. For those of you who've only seen the films, the book is structured like this:
--End of The Two Towers, after Shelob's lair: 'Frodo was alive, but taken by the enemy'.
--The entire Aragorn-etc portion of RotK, up to and including the meeting with the Mouth of Sauron at the Black Gates. The Mouth shows them Frodo's mithril shirt and Sam's sword, and we - the readers - have no idea what's happened. We know Sauron doesn't have the Ring (we saw Sam take it), but we have to assume Frodo is still a prisoner.
--A several-day timeslip to Sam rescuing Frodo, way before the shirt ever reached Barad-dur.

This is a brilliant reveal - one given to us, not to the characters. Unfortunately, the structure of the films prevents it from happening - the scene at the Black Gates is intercut with the climb up Mount Doom. This one I understand, but it's still a shame (and as I recall, Tolkien wanted the structure kept...)

-Finally, perhaps the most famous reveal of all: Darth Vader is Luke Skywalker's father (see, I can talk about things other than LotR). This is completely and utterly spoiled by the prequel trilogy. They didn't even try to keep it a secret, so if you watch the films in order, it's not a surprise in the slightest.

It's possible that they tried to put in a second big reveal for if you do watch them in order: that Palpatine is evil. I don't know, because, obviously, I knew it all along. I may have to go back and watch the prequel trilogy and pretend I don't know that Palpatine is Sidious. Maybe it's a real shock if you do that? But I don't know.

Obviously these are only a couple of examples. I'm sure you can think of more. Or maybe you think I'm silly for minding this loss at all?

hS

It's been a while since I've seen the film, but from what I remember, the ending had quite a twist - thanks to a certain iconic landmark. The prequel from a few years back, Rise of the Planet of the Apes, gives that away completely.

The identity of Luke's father was the first thing I thought of, halfway through reading your post and before I'd seen that you'd used that example yourself. As it happens, I recently read a webcomic bit that featured a character who has never seen Star Wars, and more importantly knows nothing about it, and a couple of the other characters discussed what order she should watch the films in, for that very reason.

Unsurprisingly, Star Wars nerds, uh... sorry, I mean fans, have already given this some thought and have suggested a couple of alternative viewing orders. The proposed 'machete order' (IV, V, II, III, VI) seems to make a lot of sense, and the guy argues his case very well.