I re-downloaded the kernel and set up serial console. Unfortunately
the MMU bug did not reappear on startup. However it did when shutting
down or rebooting the box. I hope other people can report their
traces.

Aug 28 10:18:30 frodo su: multix to root on /dev/ttyp0
Aug 28 10:18:35 frodo reboot: rebooted by multix
Aug 28 10:18:36 frodo syslogd: Exiting on signal 15
syncing disks... done
unmounting file systems... done
rebooting...
uvm_fault(0x26acf8, 0x3000, 0, 0x2) -> 0xe
  type 8, code [mmu,,ssw]: 802070d
trap type 8, code =3D 0x802070d, v =3D 0x3af0
kernel program counter =3D 0x47ff028
kernel: MMU fault trap
pid =3D 341, lid =3D 1, pc =3D 047FF028, ps =3D 2714, sfc =3D 1, dfc =3D 1
Registers:
             0        1        2        3        4        5        6       =
 7
dreg: 00000808 00000000 0089AFB4 008547FC 00000200 00000000 00000000 000000=
00
areg: 047FF000 40800000 0089AF44 00003AF0 FFFFBE00 007AA780 0089AE74 FFFFBD=
FC

Kernel stack (047FF6E4):
7FF6E4: 001F42DC 047FF760 00000080 0089AFB4 008547FC 00000200 00000000 0000=
0000
7FF704: 00000000 0089AF44 00003AF0 FFFFBE00 007AA780 007A8A50 00000001 0000=
0000
7FF724: 00000000 00000000 00000001 00000000 00000000 00000008 00000000 0000=
0000
7FF744: 00000000 28874267 0089AE74 00003090 00000008 0802070D 00003AF0 0000=
0808
7FF764: 00000000 0089AFB4 008547FC 00000200 00000000 00000000 00000000 047F=
F000
7FF784: 40800000 0089AF44 00003AF0 FFFFBE00 007AA780 0089AE74 FFFFBDFC 0000=
0000
7FF7A4: 2714047F F028B008 0EEE070D 43E90090 00003AF0 00003AF0 00000000 F013=
4000
7FF7C4: 047FF030 047FF02E 047FF02C FFFFFFFF 4000FF0D 0002F24C 00000000 0018=
2F2E
7FF7E4: 00004000 00000000 20200000 047FF016 00020000 00000000 00003AF0 6DB6=
DB6D
7FF804: B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6D=
B6DB
7FF824: 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB=
6DB6
7FF844: DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6=
DB6D
7FF864: B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6D=
B6DB
7FF884: 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB=
6DB6
7FF8A4: DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6=
DB6D
7FF8C4: B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6DB6DB 6DB6DB6D B6DB6DB6 DB6D=
B6DB
panic: MMU fault
Stopped in pid 341.1 (reboot) at        netbsd:cpu_Debugger+0x6:     =20
 unlk    a
6
db> bt
cpu_Debugger(8022700,8,47ff760,47ff74c,1f42fc) + 6

this happens with:
NetBSD 3.0_BETA (GENERIC) #0: Sat Aug 13 15:53:37 UTC 2005
=09builds@works.netbsd.org:/home/builds/ab/netbsd-3/mac68k/200508120000Z-ob=
j/home/builds/ab/netbsd-3/src/sys/arch/mac68k/compile/GENERIC

but I also had on disk another kernel and it too doesn't reboot properly:

NetBSD 3.0_BETA (GENERIC) #0: Fri Jun 10 16:58:09 UTC 2005
=09jmc@faith.netbsd.org:/home/builds/ab/netbsd-3/mac68k/200506090000Z-obj/h=
ome/builds/ab/netbsd-3/src/sys/arch/mac68k/compile/GENERIC

as a record the full kernel boot messages of the first kernel are:

Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 3.0_BETA (GENERIC) #0: Sat Aug 13 15:53:37 UTC 2005
=09builds@works.netbsd.org:/home/builds/ab/netbsd-3/mac68k/200508120000Z-ob=
j/home/builds/ab/netbsd-3/src/sys/arch/mac68k/compile/GENERIC
Apple Macintosh IIci  (68030)
cpu: delay factor 266
total memory =3D 73728 KB
avail memory =3D 67596 KB
mrg: 'Mac IIci/Q700 ROMs' ROM glue, tracing off, debug off, silent traps
mainbus0 (root)
obio0 at mainbus0
adb0 at obio0
asc0 at obio0: Apple Sound Chip
iwm0 at obio0: Apple GCR floppy disk controller
fd0 at iwm0 drive 0: (drive empty)
ncrscsi0 at obio0 addr 0
scsibus0 at ncrscsi0 channel 0: 8 targets, 8 luns per target
zsc0 at obio0 chip type 0=20
zsc0 channel 0: d_speed   9600 DCD clk 0 CTS clk 0
zstty0 at zsc0 channel 0 (console i/o)
zsc0 channel 1: d_speed   9600 DCD clk 0 CTS clk 0
zstty1 at zsc0 channel 1
nubus0 at mainbus0
macvid0 at nubus0 slot c: SuperMac Spectrum/8 Series III v1.1
macvid0: 1024 x 768, monochrome
macfb0 at macvid0
wsdisplay0 at macfb0 (kbdmux ignored)
ae0 at nubus0 slot d: MacCon Ethernet, 16KB memory
ae0: Ethernet address 00:00:94:03:d7:ae
fpu0 at mainbus0 (mc68882)
ae0: NIC memory corrupt - invalid packet length 65280
adb0 (direct, II series): 2 targets
aed0 at adb0 addr 0: ADB Event device
akbd0 at adb0 addr 2: extended keyboard
wskbd0 at akbd0 (mux ignored)
ams0 at adb0 addr 3: 1-button, 100 dpi mouse
wsmouse0 at ams0 (mux ignored)
scsibus0: waiting 2 seconds for devices to settle...
sd0 at scsibus0 target 0 lun 0: <IBM OEM, DFHSS2F, 4141> disk fixed
sd0: 2150 MB, 4390 cyl, 8 head, 125 sec, 512 bytes/sect x 4404489 sectors
boot device: sd0
root on sd0a dumps on sd0b
root file system type: ffs
Sun Aug 28 10:16:16 UTC 2005
swapctl: adding /dev/sd0b as swap device at priority 0
Checking for botched superblock upgrades: done.
Starting file system checks:
/dev/rsd0a: file system is clean; not checking
Setting tty flags.
Setting sysctl variables:
Starting network.
Hostname: frodo
IPv6 mode: host
Configuring network interfaces: ae0.
add net default: gateway 192.168.1.254
Adding interface aliases:
Building databases...
Starting syslogd.
Aug 28 10:16:52 frodo /netbsd: ae0: NIC memory corrupt - invalid
packet length 65280
Checking for core dump...
savecore: no core dump
Mounting all filesystems...
Clearing /tmp.
Creating a.out runtime link editor directory cache.
Checking quotas: done.
Starting virecover.
Starting local daemons:.
Updating motd.
Starting sendmail.
Starting inetd.
Starting cron.
Sun Aug 28 10:17:42 UTC 2005
Aug 28 10:17:44 frodo getty[316]: /dev/ttyE0: Device not configured

(interetsting to note that serial console didn't work, fortunately
telnet did work).