I actually lost my Instagram account because, I believe, it filled in my email field with a dummy one, user@example.com and then when I had to do verification, I could never recover the account. I believe it was in the very early days of Instagram although it's possible there was user error on my part in this case.
It is too bad because for symmetry I used the same use name in a number of places (not the one I have here).
I lost a yahoo account because I put in incorrect information (I claimed to be 99 year old female or some such thing) and then forgot the password. I never really did anything with my yahoo account though, so it doesn't matter other than I couldn't unsubscribe to some mailing list.
Lost my Yahoo account because it forwarded mail to another account and so I never logged in to it. Then Yahoo deleted it for inactivity ... No warning issued, just gone one day. So now I'm locked out of my YouTube account because it wants to send a verification code to the Yahoo address.
It is too bad because for symmetry I used the same use name in a number of places (not the one I have here).