If I found some cash (however much), I would most certainly keep it. My line of thinking is that if I didn't take it, someone else would. The thing is, if you find a bill in the street, there's not really anywhere you can take it. In a bank, it's a different story I suppose, since they deal in money and could keep it safe in case someone came in asking for it, but I think the Chaotic side of me would probably still take it.
To tell you the truth, in my experience, this has only ever happened to me once. (Seriously!) Last year at university, I was in the canteen getting my lunch, and I handed over a £20 note to pay for around £3.50 worth of food. Well, the checkout operator gave me, in change, a £5 note, a £1 coin, a 50p coin, and a £20 note. Clearly she thought the £20 was a £10. Having worked on checkouts, I'm well aware that it's an easy mistake to make, as the £10s and £20s are stored in the same compartment. Thing is, I didn't actually realize I had been given £10 too much until later in the day. By that time, I wasn't going to go back and explain the mistake. And hey, I'm a student, I'm poor! I need the money... to spend on DVDs. :D
To tell you the truth, in my experience, this has only ever happened to me once. (Seriously!) Last year at university, I was in the canteen getting my lunch, and I handed over a £20 note to pay for around £3.50 worth of food. Well, the checkout operator gave me, in change, a £5 note, a £1 coin, a 50p coin, and a £20 note. Clearly she thought the £20 was a £10. Having worked on checkouts, I'm well aware that it's an easy mistake to make, as the £10s and £20s are stored in the same compartment. Thing is, I didn't actually realize I had been given £10 too much until later in the day. By that time, I wasn't going to go back and explain the mistake. And hey, I'm a student, I'm poor! I need the money... to spend on DVDs. :D