My Facebook email for ages was my school email (as is tradition, right?) and one day someone registered as my actual email around the time I was doing a bunch of address consolidation because my school was moving all historical accounts to a separate subdomain.
I clicked to confirm foolishly (should not have done that) and it became associated with someone else's Facebook account.
Facebook has a process for this. You request an email to your address and it sends you one and you reply and it removes the email from the other guy.
Well, I did that except he set it without the '.' and when I replied from mine it wouldn't accept it. I tried again as it was and only realized after three tries what the problem was. Facebook's difference in verification processes (click to confirm / reply to dissociate) meant that I was not doing the right thing.
Repeating the action means I looked like a fraudster so that must have been why even though I added the dot version as an email to send as it would no longer accept me.
To make matters worse, I decided I'd just fix it by resetting my password and logging in and removing my email.
Well, I succeeded in the password reset but Facebook protects you here by requiring friends to verify it's you. Well, I didn't know his friends so I just let it go: he could no longer log in except via phone number (I hope, or he was locked out) and I couldn't associate my email correctly.
Then one random day I tried again and it worked.