How do solo builders test for shared state and concurrent updates in AI-generated apps?

I shipped a small tool built with Claude Code thinking it was ready to go. The moment a second person opened the app and made a live update, it wiped out my recent edits because there was no conflict handling built in.

Since AI agents only build what you specifically tell them to I now ask one question in every prompt: what happens if two users do this exact action at the same time?

How do you handle multi-user testing as a solo creator without having a dedicated QA team to catch these gaps?