It might be due to a filesystem problem.  Meherally has this solution which worked for the gentleman. “Shutdown, restart by pressing power button. As soon as you hear the chime, press cmd-s. After a while lot of text will roll down the screen, leave the keys. At the prompt, type fsck -fy. Note the space. then hit return. If at the end of the run you get something was repaired, then run it again till you get ‘OK’. After that type reboot and you will boot into regular OS X. Then do whatever you did to get this message and see if it goes away.”