I lost access to gmail because of 2FA - Google Authenticator to be precise.
One random sunny day my 2 year old bit in my phone, thereby breaking it. A few days before i had reinstalled linux and apparently had not yet logged into gmail. So suddenly I have only unrecognized devices and no authenticator. Despite living in the same place, using the same wifi, etc, I simply cannot get back in since then. Its been years with dozens of attempts from any possible 'known' device, but there simply is no way. I know the password, I know previous contacts, i have old emails, i have the password, ... But even when I enter all the info Google requests for account recovery I simply get a screen saying they will get back to me - and never do.
My fault for not having a backup sheet of codes, but I was too worried someone would find and abuse that sheet. Well, goodbye 10 years of email.