One of the most common scenarios I've seen is a simple one. You hand someone a $20 and they give you change for a $10. You point out that you handed them a $20, and they go, "oh, my bad!" and make the correct change. Obviously this is a better deal than some petty dollar scam, as they net $10 on the transaction.
Mark taught me, and I always follow this when paying by cash, to announce in a clear voice what bill you are presenting. "That will be $5.75!" the clerk announces, and I reply, "Out of a $20" as I hand her the bill, face up, in full view of the security camera. It makes it harder for them to argue that you handed them a ten later on.
And fortunately, most stores have security cameras - focused on the cashier, not you. Because while robberies and the like are not uncommon, it is far more common for the person behind the counter to be the one stealing. Either they are not recording cash transactions, or are short-changing the customers.
Of course, one way to avoid this problem is to use a credit or debit card - there is no change involved. But even then, I have seen servers play games with numbers. Often they will present a bill that is not itemized, but merely the total on the charge receipt. This makes it hard to figure out what it is they are charging you for. But I am not sure whether they can personally profit from this, as charged bills don't generate cash they can take out of the till.
But speaking of which, I never did get a receipt from that store clerk, and in retrospect, I am not sure how the total came to $8.28 even at inflated tourist prices here. Next time, I will be more careful - but of course, stopping at the convenience store isn't something I do very often.
It is funny, but I just don't use cash very much anymore. Some folks go to an ATM every day, it seems, taking out small amounts of cash for spending. I go maybe once every few months (one bank we do business with will lock out your ATM card if it is not used every three months - a pain in the ass, as you have to reset the card at the bank and have a new PIN issued!) to get some spending cash.
And when that lady handed me that change, I realized what a PITA it is to have coinage clinking around in your pocket. Coins I will have to take home, sort, and roll up and eventually take to the bank to deposit - because I am never going to remember to take the coins with me to spend, as I don't use cash anymore.
The video above illustrates how grifters can short-change store clerks and bar tenders with clever manipulation. If you get a job that involves handling money with the public, just watch out for these folks who want to do deals like this "I want to get rid of some dollar bills" or some such nonsense. It usually is just a means of confusing the cashier and setting up one of these short-change deals. As the guy illustrates in the video, do each transaction separately, and avoid handing money back and forth. Mark reports that someone tried this on him once at the store, and he was able to shut it down by keeping each transaction separate - and refusing to make change for the customer ("we're not a bank!" he told him).
I suppose it is a small-time con, short-changing people. But oddly enough, people often get more pissed off about small things like this, than they do in losing hundreds or thousands of dollars in con-job investments or raw deals on leased cars.