The Feature That Worked Perfectly Until Two People Used It at Once

I built something fast, reliable, handled every case I could imagine. It also fell apart the moment two people used it at the same time. A race condition I had never hit because I was always the only person testing it.

The launch brought the second person. And the third. The feature that had been perfect for months broke within an hour.

I had built for the only user I knew. Me. And I had never asked what would happen when there were more.

Have you ever built something that only worked because it was never actually used?