Pretty much never.

One particular terrible case I saw was when a developer left a testing flag in a build that got pushed to production and used for years. Had you set the right &whatever flag in the URL you'd have unauthenticated access to everything. It was discovered years after the fact when the software was no longer in supported status, so nothing was ever wrote up about it and told to the users. "They shouldn't have been using it by now anyway, no use in bad press and getting users worried".