I had this happen to me! I created an instance for myself and a few friends. I didn’t tell anyone about it outside the 3 of us and I didn’t post it anywhere. I had some trouble with my SMTP settings initially so disabled the email verification. I fixed the settings and re enabled the email verification. Someone messaged me pointing out that I had an extra 30,492 users. Within 2 days they found me and created that many accounts.
It took a long time to delete. Querying the DB or adding a record took milliseconds, but removing one of these users took about 6 seconds each. They must have had a lot of other records in other tables that got cleaned up at the same time? Removing that many records took me a day and a half.