Ignoring simply insane plays like hitting a 19 or 20, the most common rookie mistake is standing on 16 when it’s two 8s facing a dealer’s turned up 7.  For the love of all that is good in this world, SPLIT THEM!  You can expect to lose at least 70 cents on the dollar each time you make this play.  Of course, this won’t be a problem for you if you play by good basic blackjack strategy.