The St. Louis Post-Dispatch recently did an article on tipjacking -- waiters and waitresses illegally increasing their tips. I did some additional research.
I usually leave cash for the tip. Bad. When the waiter enters the credit card info, he just adds an extra tip for himself to the credit card bill.
It's not much better if you add the tip to the credit card slip that you sign. The tip amount still has to be manually typed into the credit card system, making it easy to a a dollar or two to the tip. Not many people match their receipts to their monthly credit card statement.
The third variation is for the waiter to add an automatic 20% tip but not tell the customer, who then adds an additional tip. This is easy for the waiter to do since most food-service computer systems have a special key or function to add an automatic 20% or more tip. (Many restaurants add the automatic 20% or more for parties of six or more.)
If you catch the switch on your bill, the restaurant will usually refund the full cost of your dinner, and maybe a free coupon. Waiters are seldom prosecuted, the restaurant do not want the bad publicity. How much can a waiter make using tipjacking?.
The Post gives examples of greedy waiters adding tens of dollars to checks but the dollar or two approach is probably more common. An extra $100 a night times five nights a week times four weeks and the waiter is taking home an extra $2,000 a month -- $24,000 a year.
Because of the ecomony, tip amounts tend to be lower, so some waiters justify the tipjacking as "we're just getting what we deserve."
Remember, there is no tipping at McDonald's!