I know you said competent, so this doesn't apply to the service I used yesterday, but it blew my mind. I lost access to TOTP for a service, but no big deal, I'm a good person who kept the backup codes. The codes are all 4 digits and the service wants a 6 digit code!

Luckily it is some lame work account that someone else can unlock to get me back in. I couldn't believe that the backup codes provided are now obsolete!