If you make an error in your blackjack bet, the casino will usually help you correct it before the game proceeds, especially if the mistake is noticed early. If you place an incorrect amount or bet in the wrong spot, the dealer will typically ask you to adjust your bet or void it. If you try to bet after the cards are dealt, the casino will likely enforce the rules by either allowing the bet to stand or voiding it entirely, depending on the situation. Generally, casinos are accommodating of honest mistakes, as long as they’re addressed in a timely manner.