The Genetic Fallacy

You can't infer from the origins of a belief to the truth or falsity of that belief. If we've established that dave's not goed a geography, and then he happens to tell you something that's true, the fact that it came from dave doesn't undermine the truth of the statement about that particular capital. Nevertheless, there's a sense in which, if you can show that dave is an unreliable source, you don't really know whether it's true or false.