Has AI ever built you a clean prototype that completely broke as soon as real users joined?

I built a quick weekend tool with AI and used it with my partner for days with zero issues. Everything was working smoothly until I invited a third user, and instant sync conflicts started wiping out data.

The AI picked the easiest, single-user setup because I didn't explicitly tell it to plan for concurrent edits. Now I never start a build without reminding the prompt that multiple people will be using it at the same time.

What default rule or constraint do you always add to your prompts to keep AI from making bad structural assumptions?