I did that before. Logged in as read only access. 80k rows updated... Uhhh shit what? Run an update script to change admin accounts email address, it runs. Change it back. So the read only access was actually full access for some reason on that server. I don't know, I don't have permission to setup the accounts.
Still don't know what those records were that got updated. It was a massive stored procedure that I had never seen before and wanted to test chunks of it. We may never know. To this day that customer often comes up with obscure issues.