The cow example sounds like a formulation of the Gettier problem[1]. Gettier wrote a single paper assailing the Justified True Belief theory of knowledge.

    Suppose that you're driving past a field and you see what you believe is a sheep. 
    It is also true that there is a sheep in the field. 
    However, what you've actually seen is a rock. 
    Behind the rock there stands a sheep.
    Do you have knowledge that there is a sheep in the field? 

This and related problems are a great starting point to a lot of interesting epistemology. I think you would enjoy contextualist epistemic theory. [2]

[1] http://alfanos.org/pdfs/04_issues_philo_fall08/07_gettier.pd...

[2] http://plato.stanford.edu/entries/contextualism-epistemology...