From these puzzles you will hopefully see that humans are quite capable of knowing something that is not true.
This only represents one facet of human fallibility, by the way. There are many others, one of which is unsolvable, as far as I can tell, and that is the fallibility of the senses. We can only perceive a representation of the world through our senses - whether this perception leads to correct conclusions about reality is only something we can know - see above for more information about that.
Well - enough preaching. Back to the Fringe.