When I built the google+ importer, we settled on setting primary email addresses to :googleid@gplus.invalid and when people logged in through google auth, it would update their email address to the address found via google authentication.
However, I happened to notice an active user with successful google authentication, with their gmail address listed with their “Associated Accounts” as “google_oauth2 (actual_email_address)” but the primary mail still being @gplus.invalid.
Edit: I discovered that the user had separately signed up with the same email address, so the address was already in use as a separate user. So unlikely to be a bug.