Secondly, I was paging through my book review notes, and discovered an old favorite that I've never mentioned here. The novel begins with a sentence something like this - "The last thing Vampy McVampire expected when he walked home from school that day was to be attacked by a werewolf." That's hokey enough in itself (and please don't start your book with a sentence about the last thing your character expected), but what's worse, within the next one page, the author proceeds to establish that his character is a vampire, walking home alone late at night, outside of his school, where over the past several days a large number of werewolf attacks on vampires have occurred. Soooooo....probably not the smartest bat in the cave if the last thing he's expecting is an attack which is clearly extremely likely.
As it turned out, the book was pretty iffy all the way through so it didn't really matter if the author lost the reader's trust right away or five pages later. But if you've written a book with integrity and continuity, and then you blow the reader's trust with a first line designed for impact but without care for how it meshes with the rest of the book...that's a bad thing. Once you lose your reader and they're not sure if they can believe you, it's nigh impossible to win them back. And it's no fun to read a book when you're not sure the author took care to ensure he actually means what he says.