As a bar manager, here's my two cents. The checksum method suggested is quite a good idea. The easiest way to discourage someone writing in a different amount is to be sure to put a dollar sign directly in front of both the tip and the total with no space between it and the first number. You'd be surprised how often that will actually stop this from happening.

And, know that this doesn't happen THAT often. I have worked in many places, and I know of it happening a few times from one person, and that person went to jail. It is a federal offense to do anything fraudulent with a credit card/debit card.

Mistakes do happen, so do call the restaurant AND your credit card company/bank. The only way the managers will know if someone has a pattern of this is if people let them know. Then something can be done. But if it was an honest mistake (I've done it) it can be easily fixed.