https://www.youtube.com/watch?v=SA6vbyPqqdg
It's the sort of mistake that amateurs wouldn't make. In fact, it's almost harder for amateurs to make this mistake.
This unfortunate dealer in a Super High Roller Cash game in Las Vegas produced a duplicate card.
Now, given that this is a $25,000 buy-in tournament, you'd expect things to run smoothly. Having two of the exact same card doesn't really work in poker.
Surprisingly, not one player noticed what was happening until after the pot was awarded, but the winnings had to be refunded after the mistake.