I'm going to start a grocery store soon, and my main concern (aside from shoplifters) involves dishonest customers out to make cashiers give more than the correct change.

Among their MOs include paying for their groceries using a small-valued bill (for example: a 100-peso bill), but once you give the correct change, they will "complain" that they gave you a larger-valued bill (say, a 1,000-peso bill) and demand for their "correct change".

This might be easier to handle when it's a lone customer, but it can become a problem when there are multiple customers at once (and where I live, it's very common for a group of customers to each say what they want to buy at the same time, causing confusion).