Full Idea
The evidence that gives me knowledge of P (there are cookies in the jar) can exist without evidence for knowing Q (they are not fake), despite my knowing that P implies Q. So closure fails.
Gist of Idea
P may imply Q, but evidence for P doesn't imply evidence for Q, so closure fails
Source
Fred Dretske (The Case against Closure (and reply) [2005], p.33)
Book Reference
'Contemporary Debates in Epistemology (2nd ed)', ed/tr. Steup/Turri/Sosa [Wiley Blackwell 2014], p.33
A Reaction
His more famous example is the zebra. How can P imply Q if there is no evidence for Q? Maybe 'there are cookies in the jar' does not entail they are not fake, once you disambiguate what is being said?