What is Mountain Lion (10.8.2) shutdown cause -81?

My 15-inch, Mid 2009 MacBook Pro running OS X 10.8.2 mysteriously crashed while it should have been sleeping. Nothing in the Console logs indicate that it was shutting down, and when it booted up it had a lot of (disk) journal logs to replay, so I'm assuming it just crashed.

The only clue as to why is a kernel log entry during boot up: "Previous Shutdown Cause: -81". Does anyone know what this is (and what to do about it)?

61% battery life left when I booted it up the next day, so it wasn't loss of power (and in the past it has hibernated when battery got low anyway).

Solution 1:

In general a negative shutdown codes are related to the hardware issues. Unfortunately the precise meaning of the codes is not publicly documented, so the best is to back up your data and give it to the repair.