Comment 2 for bug 1438036

This suggests the shell was crashing, or in otherwise bad state. Can you see in /var/log/upstart/whoopsie.log whether a crash for unity8 was uploaded around the time the alarm rang, and post here the OOPS reported?

Otherwise, please see if there is a .crash file for unity8 in /var/crash and upload it here.

Unfortunately if you rebooted the crash might be incomplete (I understand why you did!) and will make it difficult for us to track down. Please let us know if you can reproduce the issue.