This great 2011 post by Roy Rapoport tells the story of how a software company created and incrementally improved a chat-bot that collected and organized the team's coffee orders -- and how the system grew, drip by drip, into a full-fledged bank. Rapoport presents it as a cautionary tale about feature creep -- but it's also a neat parable about how all currency arises from debt, which is the thesis of Debt: The First 5,000 Years, which is one of the most provocative books I've read in years.
Remember the part where someone comes back with your drink and you find out you don't have the $4.15 you owe them? It's annoying. So after a particularly fun weekend, suddenly debts could be declared to Caffeinator. Obviously, the potential for mischief when you can say "John owes me $4.15" is high, so the first implementation required people to declare their own debt ("I owe Roy $4.15."). Of course, I also had to implement debt repayment ("John paid me $4.15"). The next implementation allowed for a proposed debt ("John owes me $4.15," which would result in Caffeinator telling you "John says you owe them $4.15. If you agree, send me a message including this key: 'xxxasdfasdf325'"). Then, of course, I got curious about the total balance of debt/credit and so had Caffeinator report your TCNW (Total Caffeinator Net Worth) -- your total credits minus your total debts.
It didn't take long to see some people's TCNW fluctuate way more highly than simply coffee ordering could account for, and realize Caffeinator became the default way in which social monetary debts were being tracked. Your coworker with Amazon Prime order something for you? "I owe Matt $88.95."
Once the network of debt and credit became saturated enough, it turned out that most people had dozens of people with whom they carried either a Caffeinator debt or a Caffeinator credit, and I came up with a way to allow people to simplify their credit/debt situation by reassigning debt. Imagine Jim owes you $10, and you owe Bob $10. Well, that's easy. Tell Caffeinator "reassign $10 from Jim to Bob" and suddenly Jim owed Bob $10, and you were out of the picture. Heck, two people owe you $5 and you owe four people $2.50 each? You can eliminate six credit/debt relationships at once.