One of our sites kept getting this error every once in awhile. After thinking it was some sort of plugin issue it turns out it had to do with the /tmp folder on our server.
It was getting filled up by eaccelerator to be precise. What is Eaccelerator?

Normally we get a warning when it is close to full and I will delete all the files when I get this notice. I guess this time I just did not heed the warning and mysql was not playing nice.

Here is how to clean out your /tmp drive via ssh.

1. Login via putty.
2. type the following.
3. cd /tmp
4. rm -rfv eaccelerator

After I did this all was back to normal.

I have also heard about people getting the same error and having to repair their database via phpmyadmin.

For us it was eaccelerator eating up space.

Next step is to set our /tmp to delete itself automatically. I will update this post on how to implement this soon.

Hope this helps a few people out there.