udeshjrWP-FB-AutoConnect has a problem of giving "Failed nonce check. Login aborted." when a user try to login through the FB login button. This error doesn't appear every time but for some users, so I believe that is a issue regards to cookies, wordpress security system and WP-FB-AutoConnect login process. I listed down some references for the readers which might help to resolve this.
Developers page on WP-FB-AutoConnect (check 20th FAQ)
http://www.justin-klein.com/projects/wp-fb-autoconnect
WordPress documentation on nonce (you will find more links there)
http://codex.wordpress.org/WordPress_Nonces
