The idea that knowledge entails certainty is a very intuitive one. It's easy to forget this, because most of us have it drilled into us, early in our epistemological careers, that embracing a certainty requirement on knowledge leads to skepticism, and we're rightly convinced that skepticsm is crazy, so we start getting used to the idea that there can be uncertain knowledge. Someone can know that p without being certain that p. If we say it enough times, it will stop sounding like a contradiction. And most of us have now said it enough times, so that it has stopped sounding like a contradiction.
Faced with a choice between skepticism and uncertain knowledge, we should indeed choose the latter. But we shouldn't forget that it's an intuitive cost. Given the choice to avoid both, we should consider it seriously.
Is this sounding familiar? It's exactly the opening structure of Lewis's "Elusive Knowledge," but discussing certainty instead of fallibility. I think the argument goes through in just the same way. Contextualists about 'knows' are uniquely positioned to vindicate that knowledge is certain knowledge, and to do it without resulting in skepticism.
The basic structure of it is easy. Let 'knows' and 'is certain that' both be context-sensitive, and let it be that for any context c, the property picked out by the former in c entails that picked out by the latter in c. But a typical effect of asking or enquiring about 'certainty' is to induce a more skeptical context. So I might answer differently to the question "do you know that p" than I would to the question "are you certain that p". But once I've answered 'yes' to the first, I face strong pressure not to say 'no' the second -- or if I do, it will feel like a retraction.
So if you're a contextualist about 'knows,' then you can, if you want, think that knowledge requires certainty. And it looks to me as if there's every reason we should want that. "I'm not certain that p but I know that p" sounds crazy.