I've also seen it happen when a system changes its email parameters, like switching from considering the TLD to ignoring it. Suddenly john@email.com and john@em

I've also seen it happen when a system changes its email parameters, like switching from considering the TLD to ignoring it. Suddenly john@email.com and john@email.net are sharing the same account, or have two different accounts and usernames linked to the same address. I had a family tech support session on this exact issue last week. A relative cleared her cookies and found her saved email and password for Facebook were logging her into a profile she'd never seen before. At some point Facebook had decided name@netzero.net and name@netzero.com were the same thing. We got around the issue by logging in using her account name gleaned from someone on her friends list.