This one is from leading epistemologist Richard Feldman (“Proper Functionalism,” Nous 27 (1993), pp. 34-50). His is a modification of one of Plantinga's own counterexamples to the sufficiency of pure reliabilist accounts of knowledge. Plantinga's counterexample to reliabilism goes as follows:
"Suppose I am struck by a burst of cosmic rays, resulting in the following unfortunate malfunction. Whenever I hear the word 'prime' in any context, I form a belief, with respect to a randomly chosen natural number less than 100,000, that it is not prime. So you say "Pacific Palisades is prime residential area" or "Prime ribs is my favorite"...; I form a belief, with respect to a randomly selected natural number between 1 and 100,000 that it is not prime. The process or mechanism in question is indeed reliable (given the vast preponderance of non-primes...) but my belief -- that, say, 41 is not prime -- has little or no positive epistemic status. The problem isn't simply that the belief is false; the same goes for my (true) belief that 631 is not prime, if it is formed in this fashion. So reliable belief formation is not sufficient for positive epistemic status." (Warrant: The Current Debate (OUP, 1993), p. 210.)
Feldman then points out that one need merely change the counterexample so that the cause of the beliefs about primes is not a burst of cosmic rays, but rather a cognitive faculty formed by an intelligent designer who designs the person to naturally and spontaneously form such beliefs about prime numbers whenever they hear the word 'prime'. In such a case, all of Plantinga's conditions of warrant are satisfied: according to the thought experiment, we have a cognitive faculty that, when functioning properly, reliably produces sufficiently firmly-held true beliefs when in epistemically congenial environments. However, such beliefs have little by way of warrant. Therefore, the conditions laid out in Plantinga's proper functionalism are not sufficient for warrant.
 Very roughly, a pure reliabilist account states that a true belief counts as knowledge if, and only if, it is produced by a reliable belief-producing process or mechanism. So, for example, consider my belief that I see a tree. That belief was formed by my visual perceptual system. If that system or mechanism or process is sufficiently reliable (i.e., most of the perceptual beliefs it produces are true in the appropriate actual and counterfactual circumstances), then (according to pure reliabilism), that belief counts as knowledge; if it's not reliable, then it doesn't count as knowledge.