Playing correct strategy reduces the dealer's edge against you at blackjack to as low as 0.5%. Even that small edge is due to one glaring fact: If you bust your hand first, and the dealer busts his hand after all the other players have acted, you still lose. This is why savvy players never hit their 16 when the dealer is showing a 6; if only five cards won't bust you, stand and, assuming the dealer has a 10 in the hole, let them take the next card and hope it busts them instead.
There is one exception to this rule, a hand which although it could be busted by taking another card, is nevertheless a powerful draw, so our second biggest mistake is:
2. Not hitting on 12 when the dealer's upcard is a 2 or 3
All the strategy cards and books will tell you to hit a 12 when you're up against a 2 or 3, but not necessarily why. Well, a 12 is actually one of the best drawing hands in blackjack. Consider: Only 4 of the 13 cards in the deck can bust you on your first hit (10,J,Q,K). That's less than one-third of the cards in the deck. That means if you take one hit on a 12, your odds of not busting are better than two-to-one! If the dealer is showing a 2, then it's in your interest to take a card and try to improve your hand, because the if the dealer has a 10 in the hole then they're in the same great drawing range as you. And that's the good news; anything smaller gives them a killer drawing hand (9,8,7,6,5) or leaves them a long, long way from busting (4,3,2,A). If the dealer is showing a 3, the same applies; you can assume whatever their undercard is, they're in the same drawing range. If the dealer is showing a 4, 5 or 6, however, you need to stand; now there are more cards that could bust them on the next hit, so it's no longer profitable to hit your 12.
So, if you have a 12 and the dealer shows a 2 or 3, you take one hit and then stop, no matter what the next card. Even if it's an Ace. Why? The same reason you don't hit a 13 against a 2 or a 3: The addition of an extra bust card (the 9) means it's no longer profitable to risk busting against the dealer's bustable hand. (I despair of players who I see hit a 12, get an Ace, ponder what to do next, then take a second hit and bust, or worse, take the dealer's bust card. They were so close to getting it right.)
By the way, this move exposes the dirty little secret of basic blackjack strategy, which advises you to assume that any card you haven't seen yet is a 10 or a face card. In actual fact, this will only be the case less than one-third of the time. (It's the same reason you don't take insurance against a blackjack when the dealer shows an Ace; the odds of him having a 10 in the hole are less than the two-to-one payout offered by the insurance bet.)
So why do we base our decisions on the assumption that the dealer always has a 10 in the hole? Well, for two reasons:
A. Any card we haven't seen yet is four times as likely to be a 10 or face card as any other individual card, since there are four of them (10,J,Q,K), all with the same point value of 10, and, more importantly,
B. As you may recall from our first installment, the math tends to work out the same even if the hole card isn't always a 10. If you have a 16 and you're looking at a 10 as the dealer's upcard, he doesn't have to have another 10 in the hole to already have you beat; anything bigger than a 7 will work, and that constitutes more than half of the cards in the deck. So if you're already beaten more than half of the time when the dealer shows a 10, you might as well assume that hole card is a 10; it's a convenient shorthand for "Mathematically speaking, you are probably toast" and acting accordingly by trying to improve your hand.
Getting back to hitting your 12s when you see a 2 or a 3 across the table: It may seem that every time you take that card, you end up busting. Almost one-third of the time is a lot. It always seemed to me that every time I hit my 12 in these situations I would bust, but after working out the math I take my card without hesitation, and relish those times I'm rewarded with a perfect 9. I love hitting my 12s. Heck, I'd rather hit a 12 against the dealer's 2 than try to decide whether to double down on 11 against the dealer's 10.