Security gaps
Secret keys in browser code. Anything your web page loads can be read by anyone. Keep secret keys on the server or in your host's environment variables.
Pages that only hide data instead of protecting it. If a page hides a button for non-admins but the server still accepts the request, anyone can call it. Every action needs a check on the server.
Databases left open. Supabase and Firebase projects need row-level security or security rules turned on, or every signed-in user can read every row.
Broken flows
Empty states that look like errors: a new user with no data sees a blank page or a spinner that never stops.
No error messages: a failed save does nothing, so the user doesn't know whether to try again.
Double submits: a slow button clicked twice creates two orders or two posts.
Lost work: refreshing or pressing back wipes a half-filled form.
Phone problems
Most people will open your link on a phone first. The usual issues are text and buttons too small to tap, content wider than the screen so the page scrolls sideways, and pop-ups that can't be closed on a small screen.
Works on my machine
Links to localhost, test API keys or a development database that only work on your computer. Emails that never arrive because the sending service isn't set up in production. Images that point to files on your laptop.
The only reliable check is someone else using your live link. That's what testers on VibeYoung do: they report each problem as a bug or UX issue, say how serious it is and whether it looks like the screen or the server, and you can ask follow-up questions.