If I notice, I give it back. I usually do a cursory check of the bills when I get back bill change... I don't check change-change. If they accidentally gave me back an extra quarter, I'd probably never notice, just toss it in my changepurse. If I got back an extra $10, now, that's something I'd catch right away. And of course I'd give it back. If I got allllll the way home before I noticed, I'll admit I probably wouldn't bother.
I worked retail for a short time and fell victim to a quick-change scam artist. He ended up walking out with more than $200 from my till, all from the way he managed to confuse me (throwing bills down, wants change, moves bills to this pile and that, recounting, and then picked up the money and left). I had a bad feeling about it as soon as he walked out so I shut down my register and asked my manager to count it and sure enough.... it was waaaaaay short. I didn't get fired (or even written up) as it was an honest mistake on my part, but they did go back through the last couple months of security footage to make sure I hadn't stolen anything, which was insulting, and worse, I was terribly humiliated by the whole affair. I consider myself a fairly smart person and I felt absolutely taken. (Which I was.)
Now, that's much different than accidentally giving the wrong change, but I still always felt personally responsible for my tills and I took pride in being absolutely 100% perfectly dead-on with my counts... so I would hope that if I had ever given the wrong change, my customer would have been honest enough to say something. I certainly would be, for that reason.
