[mindslight]

I think the closest, albeit boring, analogy to the situation is you go through the traditional staffed register at a busy time. The tally comes out a little lower than you were expecting. Since it's not higher, you pay and get out of the way. In the parking lot, you look over your receipt and see that one item out of many didn't get scanned. Now you have the choice of either just accepting the benefit from the mistake, or spending your life waiting in line again simply to redo their job better than they did it themselves. IMO it's foolish to correct their mistake by harming yourself instead.

I've also corrected cashiers on the spot, especially for more significant errors. I once had a cashier ring up a manual 20% off coupon as 80% off ($20 vs $80 final price). Going through with that transaction could reasonably be called dishonest, despite it being perfectly legal.