I stumbled on this when I logged into YouTube using the wrong password and found myself logged into an account where I had no activity/uploads whatsoever, when I distinctly remember adding favorites and uploads on my account. Apparently, this is what happened:
1. A while back I created a YouTube account with the username Foo, and uploaded, and favorited some videos on the site. When I created the account, I had linked it to an email address firstname.lastname@example.org.
2. At a later time, I created a gmail account Foo@gmail.com with a different password (password2).
3. At a much later time, I decided to create a YouTube account (by this time I had forgotten I already had one ) using my gmail credentials. When signing up, I had to put in Fuu as Foo was not available as a username, which I probably found strange but forgot about at that time.
4. I went back to using my former password by the time I wanted to login to YouTube again, so I put in login name Foo with the former password (password1) and then started favoriting videos.
5. After a while of YouTube inactivity, I one day decided to log back in, and absentmindedly put in my new password (password2). At first I panicked, I thought my account had been wiped out as there were no favorites nor uploads in my account. Then I noticed my account name Fuu. That’s when I tried logging in with my other password (password1), and sure enough, it took me to the account I was accustomed of using.
I just tried it out signing up test accounts (though I think I forgot my password after I changed again (or did I?) because I couldn’t login to my test account anymore) and the same thing happens.
The chances of this happening to 2 different people are I guess next to none. I wonder if the same happens with Flickr + Yahoo. Yahoo accounts take longer to sign up so I’m too lazy to test but my guess is yes. If I am right, I wonder what a good solution would be to prevent this from happening. If I’m wrong, I’m interested to find out what the fix is. I’m sure someone’s already figured it out with all these companies buying other companies.