I had a friend of mine point something out to me once... You should always look for this at your local poker room.
We've all seen the new deck setups like the one photographed above. Astute friend of mine believes that the brand the cardroom was using (years ago) must have sat in a warehouse/shipping container for too long, and was exposed to heat.... or the elements. He told me to take special notice of the Ace of Spades on the top of any new setup.
So what happens is that the Ace of Spades or the top card on these setups tends to get bent by the heat/elements and this one card will not lay flat on the table.
So if someone is not using a rather large card protector, the Ace of Spades would never lay flat on the table. Ace of Spades is likely a card combination that most players will play, and it's very easy to exploit when you are absolutely certain that your opponent has the Ace of Spades.
I haven't seen this defect in years (I don't play as much as I used to) but this is most certainly a thing and this is why I use a large card protector to ensure both of my cards stay flat on the felt, even though I don't really like using them.