Here’s a pet peeve of mine: when I pay for something and the clerk hands me change, and I’m handed bills first, then coins.
The best way I can think of to describe the problem is this: bills have to be manipulated with the fingers, and coins have to be held in the palm of the hand. Balancing the coins on top of the bills undermines both requirements. Before the bills can be folded and tucked into a pocket or a wallet, the coins have to be rolled off into the other hand. Sometimes the coins roll completely out of control, clattering onto the checkout counter or the floor.
If the clerk drops the coins into the palm instead, then places the bills on top, no second hand is required to stash the change. It’s easy to hold the coins in the palm while pinching the bills in half and tucking them into a shirt pocket, or into a wallet (which the other hand is now free to hold open). The same hand can then drop the coins into a pants pocket.
It seems perfectly obvious to me, and it’s all I can do not to give this lecture to every clerk who apparently hasn’t thought this through.