Suppose I believe that the widgets I see are red.  I am appeared to redly, and my visual faculties are working as most humans’ visual faculties would work. There’s nothing abnormal about the situation. Intuitively, I know that the widgets are red.
Suppose, then, that a lying factory worker (whom I don’t know is a liar, but looks like an ordinary factory worker) walks in and tells me that there are red lights shining on the widgets. Intuitively, I now have a defeater for my belief that they are red. I ought to withhold belief. If I continued to believe, my true belief would no longer be warranted (where ‘warrant’ is used to designate whatever turns a true belief into knowledge).
Note that there were certain faculties responsible for my forming belief that the widgets are red (mostly visual faculties), and there was a different set of faculties responsible for my coming to withhold belief (my aural faculties, faculties responsible for taking account of testimony, etc.).
Let’s revise the case in the following way. Suppose, instead, that when the factory worker tells me that there are red lights shining on the widgets, my aural faculties suddenly and momentarily malfunction (I get hit by a stray cosmic ray) so that I do not end up hearing what the factory worker says. I smile and nod, the factory worker smiles and nods, and then he walks away. I continue to believe that the widgets are red, guessing that what the factory worker had to say was something like, “Hello, the weather sure is nice today!”
Intuitively, is my belief no longer warranted? Or does my true belief not count as knowledge?