Check those receipts
Waiters sometimes added a few dollars to the tip her boss had written in on the credit card slip.
When you think about it, the system is easy to abuse. The servers are the ones who type in their tip to the credit card company after you've signed off on it. Her boss let the discrepancies slide; the amounts were small enough that he didn't want to bother about them.
Before she told me about this, I didn't always check my statement against the receipts as carefully as I should have. Now I do. I don't know whether servers are more honest in Baltimore (I like to think so) or they just haven't thought of this particular scam yet; but so far it hasn't happened to me. Still, it doesn't hurt to keep checking.