kernel_task filling up disk
This happened to me once. I used Daisy Disk to figure out where the disk usage was coming from.

In my case, there was a problem with the coresymbolicationd cache, which was growing indefinitely, and continuing through reboots and reinstalls of the latest Combo Update.

After deleting /System/Library/Caches/com.apple.coresymbolicationd, everything returned to normal.

Next time this happens, I recommend taking a look with that utility.