NetBSD 1.5 on a DS 5000/125. happens when I close out xdm. Here is
most of a stack trace (top is what I managed to write down the rest
is from dmesg). Problem seems to be in xconsole.

psignal+44
ttymodem+78
ptcclose+64
spec_close+1f0
ufsspec_close+1e0
vn_close+90
vn_closefile+24
closef+248
edfree+74
exit1+e4 (801fcca0,1,2000,858f3020) ra 8005c374 sz 56
sys_exit+14 (801fcca0,1,2000,858f3020) ra 801c1fa0 sz 24
syscall+218 (801fcca0,1,2000,858f3020) ra 80030fa8 sz 96
mips1_SystemCall+d0 (801fcca0,1,2000,858f3020) ra 204463b4 sz 0
PC 0x204463b4: not in kernel space
_DYNAMIC_LINK+204463b4 (801fcca0,1,2000,858f3020) ra 0 sz 0
User-level: pid 262
End traceback...
syncing disks... kdbpeek: NULL
kdbpeek: NULL
trap: TLB miss (store) in kernel mode
status=0x8ff00, cause=0x3000240c, epc=0x801bd190, vaddr=0x0
pid=262 cmd=xconsole usp=0x7fffdfd8 ksp=0xc8160938
panic: utlbmod: invalid segmap
Begin traceback...
kdbpoke+10 (0,1000d,c81609f8,c81609f8) ra 801bd70c sz 24
db_write_bytes+38 (0,1000d,c81609f8,c81609f8) ra 8003c600 sz 32
db_put_value+34 (0,1000d,c81609f8,c81609f8) ra 80040be8 sz 32
80040b98+50 (0,1000d,c81609f8,c81609f8) ra 80040aac sz 24
db_set_single_step+80 (0,1000d,c81609f8,c81609f8) ra 80040884 sz 40
db_restart_at_pc+130 (0,1000d,c81609f8,c81609f8) ra 800419a0 sz 40
db_trap+f0 (0,1000d,c81609f8,c81609f8) ra 801bd424 sz 32
kdb_trap+224 (0,1000d,c81609f8,c81609f8) ra 801c2864 sz 48
trap+788 (ff10,2424,c81609f8,c81609f8) ra 80030cf4 sz 80
mips1_KernGenException+c0 (ff15,6,400006,ffffffff) ra 80098700 sz 120
vfs_shutdown+6c (ff15,6,400006,ffffffff) ra 801eab1c sz 72
cpu_reboot+a4 (100,0,400006,ffffffff) ra 800727e0 sz 24
panic+134 (100,0,0,ffffffff) ra 801c24dc sz 48
trap+400 (fc34,30000010,0,ffffffff) ra 80030cf4 sz 80
mips1_KernGenException+c0 (801fcca0,1,2000,858f3020) ra 8007d534 sz 120
ttymodem+78 (801fcca0,1,2000,858f3020) ra 801ead78 sz 24
cpu_intr+58 (801fcca0,1,2000,858f3020) ra 8007f8f4 sz 40
ptcclose+0 (801fcca0,1,2000,858f3020) ra 0 sz 0
User-level: pid 262
End traceback...

dump to dev 21,41 not possible
rebooting...