“Their mistake, not mine” is just a nicer name for taking.

Blaming the error moves the decision somewhere else. Somebody undercharged you, or handed back too much change; their slip is real, and it is also finished. What happens next is yours: you either keep something you know is not yours or you say something.

Say it in the moment you notice, while it is still easy. One sentence at the counter takes less effort than carrying the knowledge around. If it is already too late, you can still mention it next time you go in.