This is not any secret or bug. Theymos already mentioned this when 2FA has been released for this forum. keep secure your email and bitcoin address which you will use first time on your account. if you forget email or if your account are hacked with email then you will able to recovery your account via Bitcoin address sign.
The forum is Bitcoin forum and bitcoin is a main currency for payment in the forum, from donation to Copper membership, and account recovery.
Some more guides on
Forum account: security, privacy, and recovery. Forum users need to secure their emails and if they use non-existing email for registration, they need to read this post and make change of account email.
This is something we recently started doing. If email sent to your email address bounces with an error message like, "This email address doesn't exist", then your email may eventually be changed to u...@bounces.invalid. (It's not possible for users to change their email address to something ending in .invalid, so this can only be an administrative change.) Because your old email didn't exist, somebody could've registered your non-existent email address and used that to steal your account.
I didn't particularly intend for the trust warning to appear for these automatic changes, but it's a niche situation and a bit difficult to fix, so I probably won't fix this unless several other people complain. It only lasts 30 days, after all.
If you don't want to set an email address, use something like yourUserName@invalid.bitcointalk.org; don't use a random nonsense email like y@x.com, since somebody might create that domain/email.