Well, to follow up:  It just died again:

_Debugger
_panic
Idle(?)
_mi_switch
_tsleep
_sbwait
[ . . . ]

Looks like tsleep?

Ooops...Almost forgot:

environment:

NetBSD/hp300 hp433 32meg ram, sources from yesterday (10.18)

On Wed, 19 Oct 94 19:46:45 PDT 
 Jason Thorpe <thorpej@cs.orst.edu> wrote:

 > Folks...
 > 
 > Sup'd yesterday.  Built new kernel, etc.  Now I'm getting this wierd panic...
 > 
 > trace:
 > 
 > panic: switch
 > Debugger
 > panic
 > Idle(?)
 > mi_switch
 > tsleep
 > select
 > syscall(5d)
 > 
 > It's really sporadic...Like, doesn't happen with any reliability...In 
 > fact, I booted the `broken' kernel to send off a better stack trace, but 
 > I can't get it to do it now...sometimes it's during boot, othertimes, 
 > it's after I've logged in, etc.  So, my trace is what I scribbled down 
 > earlier.
 > 
 > This isn't a bug report, really.  Just `anybody else seen this?' or `did 
 > I goof somewhere?'
 > 
 > Later...
 > 
 > --------------------------------------------------------------------------
 > Jason R. Thorpe               thorpej@cs.orst.edu                 758-2003
 > Systems Administrator            CSWest Room 5                    737-2229
 > CS Dept, Oregon State University           http://www.cs.orst.edu/~thorpej
 >                "I brought my BOWLING BALL -- and some DRUGS!"
 >                       -- ztp

--------------------------------------------------------------------------
Jason R. Thorpe               thorpej@cs.orst.edu                 758-2003
Systems Administrator            CSWest Room 5                    737-2229
CS Dept, Oregon State University           http://www.cs.orst.edu/~thorpej
               "I brought my BOWLING BALL -- and some DRUGS!"
                      -- ztp