There's a Coinstar machine at the entrance/exit to my grocery store. I always check the reject return as I leave the store. In the 25+ years I've been patronizing this store not only have I never seen anything in the reject bin, I've never even seen anyone using the machine (maybe that would explain why I never found anything).
Outside of Coinstar machines my biggest find would be a $5 bill on the ground at Disneyland (equivalent to about $100 now) in about 1958.. Being the Boy Scout I was/am I held the bill over my head, started waving it, and in a loud voice proclaimed, "I found $5, who lost $5?" My mom grabbed me and told me it wss my father's. Later I was urged to go into a Disneyland gift shop and buy something for myself, my little brother,nand two cousins who were with us. My 7 year old self was amazed how fast that $5 disappeared at the Disneyland gift shop. I think the same still holds true today, except replace the $5 bill with a $100.