This happens when there is no disk space.  Even after deleting files and making space, it still happens, so I did a reboot and it worked fine after reboot:
root@mirror:~# df -mFilesystem           1M-blocks      Used Available Use% Mounted on/dev/sda1               935843    935842         0 100% /varrun                     498         1       498   1% /var/runvarlock                    498         1       498   1% /var/lockudev                       498         1       498   1% /devdevshm                     498         0       498   0% /dev/shm
 
Wow, how did we use all that space?