The Feature That Only Worked Because Nobody Used It

I built something that worked perfectly. Fast, reliable, handled every case.

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. And the fourth. 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? And how long did it take to find out?