Here is a signup bug that hides well. Your app works when you test it, but the confirmation link in a new user's email sends them to localhost:3000 or an old preview URL. They click it, see an error, and leave. Nothing on your side reports a failure.
This often happens with Supabase Auth when the Site URL or the redirect allow list still holds a dev address, or when emailRedirectTo is never set in code. A five-minute check: sign up with a fresh address on your live domain, open the email, and look at where the link actually goes before you click it.
FlowWatch runs your real signup, login, and checkout flows on a schedule and emails you when one breaks. Try a free check at flowwatch.org.