I had a brief stint waitressing at a local Bavarian bakery and deli that was experimenting extending hours to serve sit-down dinners. One evening a particularly effusive customer left a huge tip, but it was on his credit card. I received only half of it, and no explanation. Ever since then I refuse to put tips on the card, it goes into the waitperson's apron when they pick up the receipts. Also, tax is paid on tips, but it is a formula. In theory someone could go through the receipts and calculate the tips as higher than the tax formula allows, so again, I don't document it.