The most I have ever found is a fiver, but I have a couple of stories about being given the wrong change...
Once in Co-Op, at the age of about 15, I walked in with about £20 with the intention of buying a lot of junky sweets and other 'munch' I bought about £6 worth, paid the woman with a £20, she started chatting, put my £14 change in the same hand as the £20 I just gave her and she give me £36 change. I walked in with a 20, walked out with loads of food and 36 quid. Happy days.
I went to a local Chinese take away with my mate. He ordered us both a chicken curry, come to about a tenner. He gave her a twenty, but she put the note on the counter and walked to the other side of the room to the other till. I swiped the twenty, going "Muhahaha" to my mate. She came back, give my mate his change. I asked for some Fortune Cookies, usually they are 15p each, she was like "Okay, okay...I give for free haha...yea, yea?!" I was like "Yea, sure..." She gave me my fortune cookies and I said, "Here. This is yours" waving the £20 note in her face. "No, no. For fwee I say haha"..."No...You don't understand, this is my mates money for the chicken curry." "No, is for fwee yea? Haha." This argument went on for a while, till I gave up and give my mate his 20 back. Again, walked in with a £20 note. Walked out with 2 chicken curries, 4 fortune cookies and about £30 quid. Happy days.
She wasn't to happy the next time I asked for fortune cookes, I think she realised what happened afterwards. When I asked for fortune cookies, she snarled at me and say, "You pay money, yes? Yes?" To which I replied, "No, I thought I could get away with shirt buttons..." The bloke behind me found it funny. She didn't.