Heraclitus proposed that everything was in flux always:
The Greeks before Heraclitus focused on the essence of things, its nature and being, which they deemed unchangeable. In contrast, Heraclitus said: "You cannot step into the same river twice, for fresh waters are ever flowing in upon you." This simple sentence expresses the gist of his philosophy, meaning that the river isn't actually the same at two different points in time.—It is a radical position and Heraclitus was the [first] to conceive it. He looked at everything being in the state of permanent flux and, hence, reality being merely a succession of transitory states. He told people that nothing is the same now as it was before, and thus nothing what is now will be the same tomorrow. With this he planted the idea of impermanence into Greek thought, and indeed, after Heraclitus Greek philosophy was not the same anymore.
Under such a system, it seems to me that knowledge will be impossible since as soon as you discover the truth of a proposition, the state of the universe changes and either the proposition itself changes or the facts that you used to justify the proposition are open to change. Therefore, at most, you can say that something was true at such and such a time, but it might not be true anymore.
The question isn't whether or not Heraclitus was correct in his understanding of the nature of events. Rather, does his theory of constant flux imply that knowledge does not exist?