I recently upgraded my iPhone and I didn't know that the Microsoft Authenticator app doesn't automatically include itself in the iCloud backup.
So when I got my new phone and loaded from my backup, it had deleted all of my authenticator details - one of which, is Trello.
I thought, ah, no probs - I'll edit my 2FA options in Trello.com, and then generate a new QR code through there. But, to get to the 2FA options on Trello.com, I need to generate an authenticator code. But I can't generate an authenticator code, because the account has been deleted.