For me, this only ever happens on the first login with a new user account. If I log into Unity first, leave it alone for a few seconds to finish initializing, and then log in to gnome-flashback, it works every time. Happened in Raring, too.
** Tags added: raring -- You received this bug notification because you are a member of compiz packagers, which is subscribed to compiz in Ubuntu. https://bugs.launchpad.net/bugs/1232299 Title: gnome-session-flashback with Compiz launches Unity To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/1232299/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~compiz Post to : [email protected] Unsubscribe : https://launchpad.net/~compiz More help : https://help.launchpad.net/ListHelp

