Self-checkout is either the beginning of the end for human cashiers or a great convenience that allows people to move more rapidly through a checkout line. It’s also bringing a new problem, according to a report from the University of Leicester: shoplifting.

While the self-checkout system means a lot of convenience for customers, it also bring the potential of increased shoplifting. Studying almost 12 million individual shopping trips, researchers found that users had a complete set of excuses ready to go when caught. Reports also suggested that shoplifting happened about twice as often with self-checkout lanes, coming in at around four percent over the normal two.

A key point of this study notes that, since the average European grocery store’s profit margin is about three percent, a four percent loss at self-checkout means an overall loss for the system, a disaster by any standard.

The most likely explanation here is almost certainly the correct one: shoppers believe there’s less risk of being caught shoplifting when handling their own purchases. This is a reasonable belief; even assuming a person is caught shoplifting, how could a store actively tell the difference between someone who slipped an extra item in their bag unawares and someone who did so intentionally? The two cases look identical; trying to determine motive is next to impossible.

Worse, what about those who actually do err in checkout? It’s one thing to slip something in a bag and intend to get it out unnoticed, but what about those who buy a bag of donuts and pay for one less than they really had? Are we to declare them all shoplifters?

This is an especially big problem as mobile payments come into play. With customers trusted to ring themselves up, there’s a greater potential risk of shoplifting. There’s also a greater potential risk of error, but that ends up the same way even without the malicious intent. With more stores moving toward self-checkout, and mobile payment-based checkout, we could be looking at a need to enhance monitoring systems in stores to spot shoplifters.

How do we do such a thing in a fashion that doesn’t treat the customer like a criminal before a crime is even committed? That’s unclear, but for right now, we know that this could be one big problem in the months ahead.