Worse than necro threads, NECRO ANSWERS!
A necro answer happens when someone (other than ourselves because we would never do this) types in a thread reply that had already been addressed previously, at least once.
It's like they read the OP's opening statement and fast forward to type in an entry, without bothering to read through any of the existing replies.
The thing I hate most is necro answers. When someone posts an answer that has already been given, because they're too lazy to read the damn thread.