I'm sure I've read worse but one that stands out as making me question the time I put into reading it is Out of the Dark by David Weber. I go into it expecting a military sci fi, and for the vast majority of the book that's what you get - aliens invade Earth and plucky humans resist etc etc. The aliens however have more reserves and air superiority so are slowly winning as the end of the book approaches, at which point you expect the main characters to pull a rabbit out of the hat and do something different. Except that's not what happens.

I was definitely not satisfied with this ending, even if there was some foreshadowing earlier in the book that made sense after knowing this was a possibility in this universe.