In change, that is.
What do you do?
Buy some lottery tickets.
Put a little into a local grocery store charity box.
Treat yourself to a nice bottle of wine and fresh flowers.
Enjoy the windfall. Don’t spoil it by pointless misplaced guilt.
Enjoy my unexpected bonus.
Many shops have a policy that - once you leave the shop your change is yours. It’s to cover being short changed, but it works both ways.
I would not think twice about keeping it.
Is “return the money” not an option for you folks? It does not belong to you. It is quite likely that the clerk is respnsible for any shortfalls, as well. You guys are whooshing, right?
I’d give it back, of course. I’d hate to see someone get in trouble from a mistake. Besides, the money isn’t rightfully mine.
StG
Not wishing to contribute to the possible firing of some poor slob who is forced to work for a pittance in retail, I return it. Is an extra $40.00 really worth your soul? (And, no, that is not meant to spark debate over whether you would be literally condemned to eternal fire because of $40.00. I just think a clean conscience is worth a lot more than money unless you are in dire poverty yourself and the $40.00 would make the difference between feeding your kids/paying your rent and not being able to do so.)
No. Not whooshing.
I’d keep it because if the situation were reversed, I’d be stuck with the $10. Show me a retailer that will correct mistakes in my favor and I’ll be happy to help correct mistakes in their favor.
If I catch it when I’m getting change, though, I’d point out the mistake. At that point it’s easy to correct the mistake in either direction.
In my cultural experience the guy at fault is almost certainly the owner of the shop. And even if not, the discrepancy is highly unlikely to result in a firing.
I’ve been ‘long’ changed before and actually given it back, but i’f you’ve got completely out of the shop then legally the money is yours.
Actually, I have known people who have had money returned in the circumstances you mention. You report the problem to the manager, and when they count the drawer, they return the additional money to you. (Yes, I do realize that you are forced to take it on faith that they don’t just lie and pocket the money. Nothing is perfect.)
If you claim to have gotten incorrect shorted, most places will count the register and cheerfully correct the error.
I don’t want $40 I didn’t earn honestly. I’d return it. In fact, the self checkout machine at the grocery store gave me a five instead of a one a few weeks ago, and I brought it to the clerk right away. She looked at me like I was an idiot.
But I got a Where’s George bill in return. Instant karma! 
Depends on my mood that day. I could go either way.
Either British shopowners are much more lenient than American ones or you don’t have any experience in retail, one.
Relating a story from my youth: Dad and I were heading to South Carolina to visit with his Mom for a week and make annual repairs on the house. At a gas stop, Dad gave me $10 and headed to the bathroom. The tank full, the attendant gave me a receipt and change, which I stuffed in my shirt pocket (I was maybe 9 or 10 at the time). A few miles down the road, Dad asked for the change, and discovered the attendant gave me change for $20, instead of $10. My father put the $10 and the receipt on the visor, and stopped at the same gas station on the return trip to make things right. He wasn’t a perfect man, but he knew what was right, and that’s what he taught me.
I’d give it back. No question.
In answering the OPs question I consulted my own experience of going to shops in the UK.
Most shops I went to, the guy behind the counter was either the owner or a family member or spouse of the owner. If the owner gave me too much change and I didn’t find out until I’d left the shop and walked off the street (the equivalent of getting into a car, since I don’t have one) I have no guilt about keeping it since I know the ramifications will be nil and I’m no saint (few people are in this respect)
If I noticed before such time I’d return it (it would usually be change of 20 instead of 10. £50 notes are quite rare in general money usage)
This has happened to me (although it was a twenty instead of a fifty) and I didn’t go back. I figure that by the time I leave the store it’s already too late. I guess I should feel bad for the clerk, but really, it’s their screw-up, why shouldn’t they have to pay for it? I’d expect to pay for my mistakes if I was a cashier. I would definitely say something if I noticed it in the store but since I usually just shove the bills into my pocket and run out of the store without checking I probably wouldn’t catch it.
I’d give it back.
In Germany a few months ago, a clerk gave me too much change and I tried to return it, but his English wasn’t very good and I’m sure he thought I was claiming he shortchanged me. I had to get out a pen and do the math on a piece of paper to convince him of what I meant. I like to think I squashed the ugly American stereotype in one shop that day.
I’d give it back, without question. Karma is karma.
As I said, things might be different in Britain. $40 could easily lead to a firing in the retail jobs I’ve had here though.
Well, firing isn’t going to be very likely in those situations outside of a serious breach of trust anyway so there’s no reason to even consider it.
And no need to justify or rationalize. I said I’d keep it as well so I’m not one to judge.
I’d give it back. I’m not Mr. Morals or anything, but for some reason I’m exceedingly scrupulous in those situations.