I recently read a book that was fantastic all the way to the end, and then it sucked. It wasn't that the ending wasn't logical. There was no crazy framing device (it was all a dream). It was just that the 500 pages that came before were brilliant while the last 75 pages seemed to fall apart.
I wish I could point to a specific thing that was wrong with the ending, but it was simply uninspired. Now, that night have been fine in another book, but this book had created a nuanced, layered narrator that was wild and strange and totally unreliable, and the ending felt like a betrayal to all that.
Endings aren't easy. They can't be too neatly wrapped up but they can't leave a reader hanging either. Too clever and they feel forced. Too random and it feels like the author pulled the ending out of his ass. And Deus ex Machina type endings never work. An ending must feel earned and must satisfy the reader.
In Deathday, I wanted to make sure that in the end when Ollie dies, readers weren't disappointed. I wanted to make absolutely sure that they didn't spend all 60k words of Ollie's journey hoping that he'd live. So I put that opening in to assure them that he was definitely going to die. And then I kept that promise. Readers might have wanted another ending, but they weren't surprised when they didn't get it.
I've struggled with endings in other stories and the funny thing is that killing Ollie at the end of Deathday was the easiest ending I've written yet. The endings of other books haven't been quite as easy. FML has TWO endings, and neither were easy to write.
So, while I can't fault any writer for stumbling at the finish line because I know just how hard it is, we still have to try to to make sure that the endings are as great as the beginnings otherwise the reader won't care how great the first 500 pages are. All they'll remember is how badly the last ten sucked.