I'm a fan of black library. The books may all have an obligatory battle scene, even if one isn't needed, but they are good reads. Warhammer short stories and novels have created gems such as 'who mourns a necromancer' and the ambassador books. But I think we all have to admit that there have been some...stretching, of fluff. So, what do you think the most egregious example is? Oh, and by the way, if you don't like spoilers, you are probably in the wrong thread.
I'd vote for Orc Slayer. In it, there is an ancient evil that sold it's soul to chaos before the time of the old ones that is sleeping under Karak Hirn. It possesses all of the dwarfs in the hold, leading to them all dying. Now, for anyone who follows the fluff, this means that every man, woman and child in the hold died. Seeing as how 75% of all dwarf children are male, losing any woman is a blow to the race. Losing everyone in the hold? It pretty much means that the hold is dead, and there just aren't enough dwarfs around to properly recolonize it. So, in making up a monster (whose race/powers are never really explained), Nathan Long managed to (probably inadverntely) destroy one of the setting's major cities by dealing a blow that it is impossible to recover from.
So, what do you think is the most ridiculous thing from black library?