How about this?
I log out of my main Facebook profile and create a new silly profile for a laugh. I click in the email they send me to confirm and immediately there is a message in the new profile saying that David (one of my existing friends on the main profile) has added me as a friend. I confirm his request.
Except that he never did. How could he add the silly profile as a friend when I had only just created the profile a minute earlier and he knew nothing about it?
So, full access to David’s profile when he never actually added me as a friend. Meanwhile David had disappeared as a friend on the main profile but reappeared later.
I message David from the silly profile and he replies ‘who the *@#! are you — I never added you!’.