Finally saw some of these in action last week.
A guy at caesars was betting 4 at a time (4 hands one each hand) in blackjack.
The dealer dealt and was showing a ten.
So of course she checks and announces "Dealer has blackjack." as she flips up the Ace of Diamonds.
This guy didn't even flinch as she relieved him of 100k with a single sweeping motion.