A concurrent sign-up is refused, not 500'd
Two sign-ups racing on the same address both reached the insert; the loser got a 500 that looked like an outage during a signup the database had just accepted.
Two sign-ups racing on the same address both reached the insert. One won; the other got a 500. From the losing side that looks like an outage during a signup that, from the database’s side, had just succeeded.
It now loses cleanly with a refusal that says what happened.