It doesn't matter what I do, create new SSH keys, add them manually to Heroku, delete all keys from Heroku and add a new one, force delete the entire ~/.ssh folder, overwrite id_rsa keys, create ~/.ssh/config file, create a keygen with -C flag, create a dsa key pair instead of rsa, deleting the keychains from the Keychain Access utility in Mac OS X, and I've just about every SU question on this.
But I keep always getting the same error message, with the same fingerprint when I run
$ git clone firstname.lastname@example.org:app-name-0000.git -o heroku
Warning: Permanently added the RSA host key for IP address 'x.x.x.x' to the list of known hosts. ! Your key with fingerprint 48:dd:XX...XX is not authorized to access app-name-0000. fatal: The remote end hung up unexpectedly
I remember having this problem some time ago with Github and gave up. I really stumped as to what to do. (Reinstall Mac OS X?)