Unexpected soft update inconsistency

[walt]

I've been seeing 'write error' while compiling firefox.  After
each write error I reboot and run fsck which produces the same
error each time:

LINK COUNT DIR I=2 OWNER=root MODE=40755
SIZE=512 MTIME=[date] COUNT 21 SHOULD BE 22
LINK COUNT INCREASING
UNEXPECTED SOFT UPDATE INCONSISTENCY

I tell it to correct the error and all seems fine until the
next 'write error' while trying to build firefox.  The error
always occurs just after the message 'leaving directory <foo>'
so it's not entirely random the way an overheating CPU might
behave.

Any ideas how I can debug this?

Anyone else seeing the same thing?