It's pretty common for restaurant servers to make mistakes -- sometimes intentional -- when ringing up a bill. Since you typically write int the tip when you sign and you never see the bill again, it's easy for dishonest servers to bump up their tip, and most customers will never notice! Unless you go through your credit card statement and keep your receipts you'll have a hard time catching these thieves... but now there's a simpler way! Checksum your tips..
Adjust the amount of the tip so that the numbers in the final total to the left of the decimal point add up to the right-most digit. In this case, the total has a “51″ to the left of the decimal point (A). 5 + 1 = 6, so the final digit should be six. Adjust the total to $51.86 or $51.96 (B) by adding nine or subtracting one from the tip (C).
This way you don't need your receipts at-hand to glance through your credit card statement and catch most tip fraud. If you see a checksum that doesn't match, dig out the receipt (you keep all your receipts, right?) and call your credit card company to report the fraud. Then call the police.
(HT: GeekPress.)
It's better still to leave tips in cash -- and always let the waiter see you pocket your copy of the receipt!