ffs driver failure ???

[johansz_(_at_)_free_(_dot_)_fr]

Hi all
After a fresh install of  obsd30 on an i586 i have still have strange
problem when uncompressing a *.gz file the machine reboots after showing
for one or three seconds the message ffs_valloc ... on the screen
At the next boot i saw in the messages file:

Feb  8 02:04:27 obsd30 savecore: reboot after panic: ffs_valloc: dup alloc

I have this one since i installed obsd 30 i never had problem with
the previous versions on this machine...
Could someone tells me what is wrong ...?
Thanks by anticipation