So what are you going to do, MeRight? Return the money that's not yours, or keep it?
I think it would be morally wrong to keep the money. My conscience would bother me, and it wouldn't make any difference whether or not I thought the shop assistant would get into trouble over the missing cash.
If that makes me a do-gooder, then I am happy to be one. If I found a fiver on the street, I'd hand it in to the police station - in fact, ds3 once found a £20 note on the street, and we handed it in - and then when no-one came forward and claimed it, ds3 got to keep it with a clear conscience.
I've been on the other end of this sort of thing too. When I was a student (and very short of cash) I took £10 out of the cashpoint - only I forgot to take the note - I took my card and walked off, completely forgetting that my money hadn't been dispensed yet (cashpoints were a fairly new thing at that point). The lady who used the cashpoint after me realised what had happened, and handed the money in to the bank, and they were able to see who had used the cashpoint before the lady who handed the money in, and repaid the £10 into my account. I never knew who the lady was, but was very grateful for her honesty.
I guess there are a number of people from this thread who would have kept my money and not given a damn about me. I'm glad none of you were behind me in the cashpoint queue.
I do have to say, though, that the people who have been treated so rudely when they did try to tell a store that they'd been given too much change were in the right to keep the money - they'd done their best to return the money, and the store lost out by not listening politely to their customer.