What are the worst or most disappointing endings in science fiction/fantasy novels

John Ottinger asks:
What are the worst or most disappointing endings in science fiction/fantasy novels? Why?
I’m going to limit myself to just one…


My nomination, and it pains me to do it, because I like the novels so much otherwise, is Peter F Hamilton’s Night’s Dawn Trilogy.
Big scale space opera, lots of cool technology, returned dead plaguing human space. Hamilton thinks big, writes big and loves the cast of thousands with viewpoint characters spread across a wide swath of locales and situations. Peter F Hamilton is a leading star of the “New Space Opera”.
But the ending, Peter, the ending! The novels are let down badly by the denouement. The denouement of the trilogy is, unfortunately, a complete and literal deus ex machina. Joshua Calvert literally finds a lost God (a naked quantum singularity) to undo all of the damage (and change the nature of human space in the bargain). I felt cheated by this. After thousands of pages, the book ends like a bad medieval morality play.
I am very happy that subsequent novels from Hamilton have had much better endings, but this series just fails on that level. I wonder if Hamilton rewrote the novels today if he wouldn’t be able to do it better. (He could hardly make it worse!)