[comp.sys.sun] panic: hat_unload -- What's this mean?

jjb@zeus.cs.wayne.edu (Jon J. Brewster) (01/24/89)

We have a 3/180S running 4.0 -- I've begun seeing the following
message (three times today):
|Jan 18 16:26:07 zeus vmunix: panic: hat_unload - pmg not kept
|Jan 18 16:26:07 zeus vmunix: syncing file systems... [1] [1] done
And then it reboots.

Anyone know what this means?  For that matter, is there a list of such
panic messages and their interpretations?