That sort of error would be hard to make at this table. When it's the player's turn, the dealer's buttons don't work until the player makes a selection. To hit a 20, the player would have to press the hit button, then the dealer has to confirm it.

In my first round on the table, I hit a 14 and got a 4. But I didn't see that I got a card, thought the button hadn't registered, and pressed "hit" again. The dealer looked at me funny and asked if I was sure I wanted to hit an 18.