I travel a lot.
Not all countries have the same standardization and cash controls as the United States or Western Europe. In most of the world, actually, everything is pretty ad hoc.
Part of this means, at least 5-10 times per year I'm having someone hand me the wrong amount of change or otherwise screwing up billing pricing.
Funny enough, no one ever hands me too much change. No one ever accidentally marks a bill as paid that isn't paid.
But the reverse happens. Often.
There's basically no adverse consequences to the person doing this. They ask you for more money or give you too little back, and they just blush sheepishly if you call them out.
What I do is calculate the change I should get before it's handed to me, and then it's easy to check. Also, I'm especially careful when someone hands me a weird mix of bills (if I'm getting back 10's and 5's at all when the change is $81, something might be up).
I actually don't mind the mental overhead of calculating before getting money - being able to do basic arithmetic fast is probably the most useful math skill for everyday life, and it keeps your arithmetic sharp.
Sometimes I wonder if there's a way to implement some kind of deterrent to doing this sort of thing, but I can't think of a way that's not asinine. So I keep doing the math before getting the change, pointing out that three 20's and one 10 doesn't add up to 80, and puzzling afterwards about if there was anything else worth doing there.