Has this ever happened to you?
Lets assume you order the following and your tab looks like this:
Eggs - 5.50
Bread - 1.25
Ham - 2.75
Milk - 2.20
Tax - 1.15
Well, unless you really scrutinized the bill you wouldn't realize (at least not easily) that you're being overcharged by $1. The correct total should be 12.85.
It's hard to catch at first glance because it looks "right" and if the server is caught, they just brush it off as a math error and come back, innocently, with the correct bill.
This can happen (quite often from what I'm told) whether the bill is handwritten or machine printed.
Have you ever caught something like this?