Related to the inaccurate covers one, I get really annoyed when a book series gets released with new covers because it's been made into a film or TV series... and they put characters who were created for the TV show on the covers!
The two worst offenders are:
Vampire Diaries: the show completely shuffled and redrew the main female characters, not a single one of them is unchanged and they mixed and matched a lot of their appearances, skills and personalities. Also Elena doesn't have a brother in the books, she has a sister who's only about 8, yet I've seen books with Jeremy on the front
True Blood: The character Jessica does not exist, (BOOK SPOILERS!) Lafayette is killed at the beginning of the second book and the antagonist MaryAnne does not exist as such (there is a Maenad in the second book, but she isn't the killer Sookie's looking for, she's much less plot relevant and she's never named), but sure enough, all have their own covers.
I have absolutely nothing against changing things in an adaptation, but tricking people who've seen the show but not read the books feels dishonest, and they might be put off a perfectly good series once they realise they've been tricked.