"God's mistakes" is a human's opinion, makes no sense.
In the minds of theists, why is it that humans are never qualified to criticize gods but always qualified to praise them?
That's not true. Well, not entirely. On a tangential note: Why, when we discuss a matter, are we supposed to address the issue at hand, rather than attack the person presenting the matter?
Anyway, back to your question. Politesse. We've all fucked up a recipe or 2 in our lives, so we don't harshly criticize the cooking of another unless we are complete assholes or we have a purpose in doing so. It's not like someone couldn't teach people better than the asshole in Hell's Kitchen. It's just that the guy isn't teaching people- he is weeding out the talented ones from the non-talented ones. He's not teaching them to cook- he's testing their cooking ability, and their ability to deal with assholes.
Unfortunately, the guys multipronged approach, due to him being an asshole, weeds out those who are not asshole tolerant, as well as bad cooks.
If he were not such an asshole, he would be able to perfectly test someone's cooking ability. Since he is also inadvertently testing for their ability to deal with assholes (because he is an asshole), he might weed out the very best cooks, simply because they cannot deal with him being such an asshole. His asshole comment for this would be "if they can't deal with assholes, they aren't a great cook", which is exactly what a great asshole would say, but not what a great cook would say. An asshole expels excrement, not goods.
Not that someone who isn't talented or good enough for heaven isn't going through the asshole gate when they're weeded out. So really, the last judgement is the hole you come out of...