Last year, I accidentally only got charged for one item instead of two when I was in a shop. I only realised when I got home, so I went in the next day and said "I need to give you money". The shop assistant thought that I was completely daft for voluntarily going in there and giving them money when I could quite easily have done nothing at all and nobody would have known. But *I* knew, so that was the important thing as far as I was concerned.