I have seen this bug on my house NFS server (a DigitalServer 3305, 
nee AlphaServer 800 5/500), which is running:

NetBSD 1.5ZC (ALPHA-$Revision: 1.161 $) #4: Sun Apr 28 12:22:46 PDT 2002
     root@digital.clock.org:/usr/obj/sys/arch/alpha/compile/DS3305
DIGITAL Server 3000 Model 3305 6500A, 500MHz, s/n NI82407HTJ
8192 byte page size, 1 processor.
total memory = 512 MB
(2096 KB reserved for PROM, 509 MB used by NetBSD)
avail memory = 364 MB
using 16300 buffers containing 127 MB of memory
mainbus0 (root)
cpu0 at mainbus0: ID 0 (primary), 21164A-2
cpu0: Architecture extensions: 1<BWX>
cia0 at mainbus0: DECchip 2117x Core Logic Chipset (ALCOR/ALCOR2), pass 3
cia0: extended capabilities: 21<DWEN,BWEN>
cia0: using BWX for PCI config access
[...]

When it shows up, NFS will begin to fail, and the system must be 
rebooted to make everything happy again. I tend to see it after 
running for about six months or so. I also believe it's a namei cache 
issue of some kind.

	Erik <fair@netbsd.org>