Ask HN: Whats the coolest thing you've ever built in 24 hours?

We ran short of paper in our Kniffel (well known dices game in germany) games in holiday. So i developed a digital version in around 8 hours. https://diesunddas.arachno.de/knoffel.php Renamed it to „Knoffel“ for Copyright issues.

CRM dashboard for a client demo the next day. Kind of CRUDy application more or less. But I understood the business workflow and had critical displays all more or less working as a proof of concept.