Punny Money has a neat, simple trick for protecting yourself from restaurant tip fraud, which is when a waiter will change the numbers on your credit card receipt in order to increase his tip. The best way to prevent it is to match all your monthly receipts to your statement, but you can use this simple checksum technique to scan a statement and quickly spot any suspicious transactions without referring to your receipts.
Figure your tip as you normally would, then adjust it up or down just enough so that when you total the bill with the tip, the last digit in the total is the same as the sum of all the digits to the left of the decimal. Here are some examples of totals that have had checksummed tips added (and be sure to read the original post for more details if you’re confused).
It won’t protect you 100%, especially if your waiter happens to know the trick and is willing to put the work into changing the receipt in a way that will keep the checksum valid. But it’s a good way to add an extra layer of protection against this sort of theft. In fact, we imagine you could create your own checksum rule if you don’t want to use one that’s made its way around the blogosphere.
“Fight Thieving Restaurant Servers With Checksum Tips” [Punny via Boingboing]