What's your favorite, most creative, most fun way you've used chance or randomness in your world?

For example, I have a "lottery" system in my world where everyone has a unique genetic code, and at a certain age (in this case 17), you're required to get a blood test for this very purpose. It's a fun way to do pair bonding, because you don't really have any choice in the matter, but the system works. There's no divorce, so people really stick with their matches, and since everyone's genes are unique, there's no possibility of incest. It's also a good way to create a system where everyone knows who they're genetically compatible with, so they can look for romantic partners who match them, increasing the chances of them having a successful relationship.<br><br>[Unrelated to the example, just an example of how this system has been used in my worldbuilding]