This does happen somtimes. It may be a mistake, maybe not. Where I work if a customer calls and reports it, the manager will research the ticket and ask the server whats up. MANY years ago a bartender got fired for adding $3.00. But it can be a mistake. Your looking at one ticket as you close another one and put the tip on the wrong one. So again it could be a mistake. Now on the other hand... But do call the business and let them know. As someone else stated you may get a free coupon for a return visit out of it. And always draw a line through the tip area or put a 0 in it so no one can add to it. Also if the machine puts your entire credit card # on the recept, black out all but the last four #'s. Next year this will be law and all machines will have to be adjusted to print the last four #'s only.