Although basic, you’d be surprised how many people don’t understand that the object of the game of blackjack is to beat the dealer, and not to try to come as close to 21 as possible.
This means you should often stand and allow the dealer to go bust or take the hits that will hopefully put him over the magical 21.
In sports, you could call this a "back-door" cover, but who really cares what it’s called. A win is a win, so collect your money and play another hand.
Another common mistake is when one of the players at your table doesn’t play "basic strategy" and hits on "weird numbers" like 16 or 17, no matter what the dealer’s up card shows.
"Oh, no. You’re going to take my card," is often heard, especially from the last players (often called third base), who want to see as many cards as possible to help influence whether they hit or stand.
But the math doesn’t bear this out, especially in these days of multiple decks. If that silly player draws "your" card, the odds of your winning are not diminished. This is even true at a single-deck game, although the odds will change and make the playing decision slightly weaker.
It’s similar to a roulette wheel, where probability might change, but the odds never do.
If you’re still unsure, try finding a table at a half-empty casino and play one-on-one with the dealer. Mathematically, it should make no difference, but at least this way you have no one to blame except yourself should you lose.