Depends how hard it is. If it's face to face, sure. Even if they're rude.
If it's like a drive thru and I've driven a kilometer before realizing they gave me an extra burger, I'll just keep it. Also if it's something like an ATM machine giving out a small bit of extra money.
The most extreme scenario I had was paying off my deceased father's credit card interest. We tried all kinds of ways to pay it off, including meeting with the person in charge of collecting the debt (we had 3 days of meetings and negotiations), transferring it from his acct, so on.
At some point we're like.. if they don't want the money, why are we the ones trying so hard to pay it back?
I kinda had the same issue with some of my clients. Like they'd lend me a test device. Then they fired the project manager I was working with. When I brought up the test devices, that test manager would be like "Oops, I forgot. I'll email you the bill." And even after a few times bringing it up, they still forget. The former project manager had to wink really conspicuously before I got the hint that I should keep it.
Overall, I have a few thousand dollars worth of unreturned stuff.