It's not "believing they are always bad". It's "being unable to determine whether they are good". If you simply don't know enough to evaluate a source, it is co

It's not "believing they are always bad". It's "being unable to determine whether they are good". If you simply don't know enough to evaluate a source, it is correct as a matter of epistemology to view its contents with a large dose of suspicion. Any historian will tell you this; a pretty sizeable aspect of the study of history is the weighing of sources, working out why they said what they did, what they're not telling you, and what they're wrong about - because every source is incomplete, biased, and contains simple factual errors. I might sit back from a book and go "wow, that was good", and it's possible for a book to be "great but wrong" - but I can only reasonably conclude that a source's contents are correct if I have got more evidence for its thesis than just that one source. So the situation is bad enough when I am encountering a new source. But if I'm already familiar with a source, and it's consistently wrong about the things I know, then I can only be even more suspicious of everything else they say; and sadly, an awful lot of publications do fall into that category. Finding a mistake in a book does make it more likely that it contains other mistakes, and it does make any given fact in the book more likely to be wrong.