So what if they fail?

I had an programmer that didn't know that. I went to the whiteboard, explained it, and now she is perfectly competent at it (and the rest of her job). What's the problem?

Where do we get the idea that if someone doesn't know some arbitrary fact that they are incapable of learning it? Hire people that are smart, have good work ethics, and play nicely with others, and everything else will take care of itself.