On my startup, I realized I had hundreds of orders a day after launch and no system to handle them properly (I expected about 3 orders max). There was no shopping cart; it was just users ordering several items separately and I had to match them by some common identifier like phone number.

Within hours, I hacked together an app that would assemble those orders into a form of a cart, and then copy-paste text. Then we'd SMS or WhatsApp the person ordering them to confirm the order.

The system would also track delivery status, payment status (no payment gateway, all cash), and whether the order arrived. It had a remarkable amount of responsibility and small margin of failure for something built in less than a day.