kernel diagnostic assertion "bp->b_bcount <= todo" failed

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
last night I got this on a netbsd-6 server running a dump(8) against a 
dk device (a wedge):
panic: kernel diagnostic assertion "bp->b_bcount <= todo" failed: file "/home/bouyer/src-6/src/sys/kern/kern_physio.c", line 176 
cpu11: Begin traceback...
kern_assert() at netbsd:kern_assert+0x48
physio_biodone() at netbsd:physio_biodone+0xe6
biodone2() at netbsd:biodone2+0x8d
dkiodone() at netbsd:dkiodone+0x84
biodone2() at netbsd:biodone2+0x8d
bdev_strategy() at netbsd:bdev_strategy+0x47
spec_strategy() at netbsd:spec_strategy+0x3b
VOP_STRATEGY() at netbsd:VOP_STRATEGY+0x33
dkstart() at netbsd:dkstart+0x2f
dkstrategy() at netbsd:dkstrategy+0xd6
physio() at netbsd:physio+0x24f
cdev_read() at netbsd:cdev_read+0x5d
spec_read() at netbsd:spec_read+0x1cd
VOP_READ() at netbsd:VOP_READ+0x37
vn_read() at netbsd:vn_read+0xa0
dofileread() at netbsd:dofileread+0x71
sys_read() at netbsd:sys_read+0x62
syscall() at netbsd:syscall+0xc4
cpu11: End traceback...

It's not the first time I see this panic, but I can't reproduce it on demand
(the previous one was in august, while backups are running every night on
several filesystems)
Does it ring a bell to someone ?

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--