True, but still: a lot of (big) businesses are still quite unhappy if they didn't pay what they should've paid. It also creates an uncertainty for them, there's often various years where the money can be requested back. Plus as far as I know, if you notice an error there's various financial rules you need to follow. You cannot just treat it as "cost saved". As a result, it is way easier to notify the other party and get a mistake fixed.