I have a MacBook Pro with Retina Display (early 2013) running OS X 10.10.1, but this issue has occured as early as the opt-in beta releases of 10.10. I have done fresh reinstalls several times.
After running for some time, (last time this issue occurred I had just about four days of uptime,) the system will completely hang. I hear a repeated sample of whatever music/audio was playing (if any), and after a few moments the computer shuts itself off.
When restarting, during the first boot the system will suddenly reboot with about 1/4 progress on the boot progress indicator. The second boot succeeds.
After restarting, the syslog shows nothing unusual, just normal output suddenly followed by the usual startup output (and nothing about the first failed reboot mentioned before).
How can I diagnose and potentially fix this issue?