The Short Change Con: Production and Prevention

In the 21^st century con artists and their devious methods are getting far and far more common. One of the largest hand to hand exchange cons is an old, yet effective con known as the "Short Change". This is an act of social engineering that allows the con artist to abuse the pressure a store cashier has during the evening rush, or tricking a simple minded person at a local mall into giving up a free $10 (in our example, any amount can actually be extracted). First we will learn how it is performed, and then we will learn how it is prevented.

In this con, the con artist is actually going to trade $20 of his own for $30 of the cashier's money. The main aspect is to bring the cashier out of their normal routine by having them hurry to meet the demand of the people in line. When a cashier normally makes change, they will accept your money and place it in their drawer. Then count out your change. However, having to hurry the cashier will exchange the cash hand-to-hand, minimizing the time needed to open/close the drawer and count the money.
Scenario:
You are in a busy line at a local Wal-Mart where the cashier feels the pressure.

Con Artist: "Hey, can I get some change for my $20"
Cashier: "Sure" >Exchanges a $20 bill for a $10, $5, and 5 $1's
Con Artist: "Wait wait, what was I thinking. I don't need all these one's, can I have a ten please?"
The Con Artist hands over what appears to be $10 in $1 bills. However, he double counts one of the bills handing over only $9. The pressure from the line causes the cashier to hurry a little more than usual, exchanging the money hand-to-hand.
Con Artist: "Um make sure you count that...don't know if it's right I'm in a rush."
Cashier: "Sir you gave me only $9"
Note: The cashier is short $1 at this point. The con artist pockets that $10 he exchanged for the $9. The $10 is in his pocket, out of sight.
Con Artist: "Sorry about that... um you have $9 in your hand already, can I just give you a dollar more to make it $10 then give you two fives on top and just get a $20 back?"
Cashier: "Sure"
The con artists hands over 11 more dollars to the cashier, she already has $9 in her hand, for a total of $20. The cashier then gives the man his $20 bill.
Makes sense right? She has 9 + 1 + 10 = 20, so she gives him a $20.
Con Artist: "Thanks, bye."
Con Artist (to himself): "My only crime is of out smarting you."
What really happened?
The cashier had $9 in her hand, however this was her money. The con artist acted as if it was his money, adding it into the counting of the change.

Prevention:
The cashier was foolish to be allowed to be taking for a free $10. When dealing with money, always remember key principles:

Pressure: Time can put on the pressure hard, however always consider risk vs reward. The risk of hurrying almost never out weighs the reward. Take your time, think carefully. Breath.
Cashier rules of conduct:Every transaction must be held as one transaction to minimize errors. When someone asks you do perform multiple actions, handel them one at a time. For example, when you give change for a $10 and the person asks for another exchange in the middle of the first one: stop, finish that transaction, close your drawer, reopen it and perform the second transaction as a totally different one.
Rule of money: Always double count your money. Even if you watch someone count it out for you, double check it yourself. Second guess yourself and believe only the facts, never assume.
Foul is fair, fair is foul: The average man is no competition to the above average social engineer. A cautious man is what every con man fears.