The Bug... (my device didnt have the serial number populated unlike others with the same issue) upgrade to 10.11 over Yosemite, everything's fine for weeks. go to log into imessage, wont take my log in... discover that any icloud service that wasnt logged in prior to the upgrade wont allow log in. as part of trouble shooting, log out of all icloud services including itunes, app store on the machine... still wont accept ANY valid apple id log in. (all other devices associated with apple ID are totally fine, its just the macbook air) go through the 1 million pages of tips and trouble shooting on apple support for this exact problem. all end with "do clean install". cant download 10.11 if you're not signed into app store. download 10.11 w utorrent. initiate clean install. OS asks for icloud login which then obviously fails, unable to start clean install. smash 2010 macbook air over right knee, open front door, discharge laptop onto front yard. new macbook air ordered, guess i was due anyways. you win apple.