I managed to get it again by running "file" on a file in an MSDOS  
mounted vnd disk image.

panic: buf mem pool index 23
Stopped in pid 9119.1 (file) at netbsd:cpu_Debugger+0x4: leave
db{0}> bt /l
cpu_Debugger(6,1,0,ce12c374,0) at netbsd:cpu_Debugger+0x4
panic(c083ae8d,17,ce12c3b8,282,1) at netbsd:panic+0x141
geteblk(ce1bfc5c,0,1,c08dbf60,6) at netbsd:geteblk
getblk(d0468844,a,0,0,0) at netbsd:getblk+0x28b
bread(d0468844,a,0,0,ffffffff) at netbsd:bread+0x48
pcbmap(db9e1d80,3,ce12c650,0,0) at netbsd:pcbmap+0x147
msdosfs_bmap(ce12c4cc,c3e1078c,0,c3e1078c,800) at netbsd:msdosfs_bmap 
+0x46
VOP_BMAP(d0468db4,3,0,ce12c66c,ce12c650) at netbsd:VOP_BMAP+0x3e
genfs_getpages(ce12c6b0,0,ce12c6d8,c03c4611,d0468db4) at  
netbsd:genfs_getpages+0x10a4
VOP_GETPAGES(d0468db4,0,0,ce12c710,ce12c794) at netbsd:VOP_GETPAGES+0x50
ubc_fault(ce12c8ac,cdb3c000,ce12c86c,1,0) at netbsd:ubc_fault+0x11b
uvm_fault_internal(c09355c0,cdb3c000,1,0,c0305a5b) at  
netbsd:uvm_fault_internal+0x76e
trap() at netbsd:trap+0x149
--- trap (number 6) ---
i486_copyout(cd54a930,cdb3c000,805e000,2000,0) at netbsd:i486_copyout 
+0x3d
uiomove(cdb3c000,2000,ce12cb90,c03fa404,d0468e3c) at netbsd:uiomove+0x6e
msdosfs_read(ce12cb08,0,ce12cb2c,282,ce12cb18) at netbsd:msdosfs_read 
+0x23d
VOP_READ(d0468db4,ce12cb90,0,cebfd3e4,d0468db4) at netbsd:VOP_READ+0x2f
vn_read(cebfe2a8,cebfe2d4,ce12cb90,cebfd3e4,1) at netbsd:vn_read+0x83
dofileread(cd54d7a8,3,cebfe2a8,805e000,40000) at netbsd:dofileread+0x94
sys_read(cd54d7a8,ce12cc48,ce12cc68,0,804e000) at netbsd:sys_read+0x92
syscall_plain() at netbsd:syscall_plain+0x1a5
--- syscall (number 3) ---
0xbbb1a587:
db{0}> mach cpu 1
db{0}> bt /l
netbsd:cpu_switch+0xda:


(gdb) target kcore netbsd.2.core
panic: buf mem pool index %d
#0  0x7f228000 in ?? ()
(gdb) bt
#0  0x7f228000 in ?? ()
#1  0xc04a8bff in cpu_reboot (howto=260, bootstr=0x0)
     at /usr/src/sys/arch/i386/i386/machdep.c:862
#2  0xc01ba6fc in db_reboot_cmd (addr=-1064239744, have_addr=0,
     count=-1071923375, modif=0xce12c0f0 "\213?\220?4")
     at /usr/src/sys/ddb/db_command.c:750
#3  0xc01ba358 in db_command (last_cmdp=0xc08c9a5c,  
cmd_table=0xc06caa60)
     at /usr/src/sys/ddb/db_command.c:508
#4  0xc01ba648 in db_command_loop () at /usr/src/sys/ddb/db_command.c: 
296
#5  0xc01bd103 in db_trap (type=1, code=0) at /usr/src/sys/ddb/ 
db_trap.c:101
#6  0xc04a5485 in kdb_trap (type=1, code=0, regs=0xce12c308)
     at /usr/src/sys/arch/i386/i386/db_interface.c:226
#7  0xc04b405b in trap (frame=0xce12c308)
     at /usr/src/sys/arch/i386/i386/trap.c:313
#8  0xc010c0b6 in calltrap ()
#9  0xc0420795 in panic (fmt=0xc083ae8d "buf mem pool index %d")
     at /usr/src/sys/kern/subr_prf.c:243
#10 0xc044509c in allocbuf (bp=0xce1bfc5c, size=0, preserve=1)
     at /usr/src/sys/kern/vfs_bio.c:513
#11 0xc044541b in getblk (vp=0xd0468844, blkno=10, size=0, slpflag=0,
     slptimeo=0) at /usr/src/sys/kern/vfs_bio.c:1087
#12 0xc044556c in bread (vp=0xd0468844, blkno=10, size=0,  
cred=0xffffffff,
     bpp=0xce12c458) at /usr/src/sys/kern/vfs_bio.c:571
#13 0xc02feb47 in pcbmap (dep=0xdb9e1d80, findcn=3, bnp=0xce12c650,  
cnp=0x0,
     sp=0x0) at /usr/src/sys/fs/msdosfs/msdosfs_fat.c:230
#14 0xc0302d56 in msdosfs_bmap (v=0xce12c4cc)
     at /usr/src/sys/fs/msdosfs/msdosfs_vnops.c:1714
#15 0xc0456f5e in VOP_BMAP (vp=0xd0468db4, bn=3, vpp=<incomplete type>,
     bnp=0xce12c650, runp=0xce12c670) at /usr/src/sys/kern/vnode_if.c: 
1296
#16 0xc045ad54 in genfs_getpages (v=0xce12c6b0)
     at /usr/src/sys/miscfs/genfs/genfs_vnops.c:796
#17 0xc04570f0 in VOP_GETPAGES (vp=0xd0468db4, offset=0, m=0xce12c710,
     count=0xce12c794, centeridx=0, access_type=1, advice=0, flags=6146)
     at /usr/src/sys/kern/vnode_if.c:1560
#18 0xc03ab1cb in ubc_fault (ufi=0xce12c8ac, ign1=3451109376,  
ign2=0xce12c86c,
     ign3=1, ign4=0, access_type=1, flags=66) at /usr/src/sys/uvm/ 
uvm_bio.c:279
#19 0xc03ad9be in uvm_fault_internal (orig_map=0xc09355c0,  
vaddr=3451109376,
     access_type=1, fault_flag=0) at /usr/src/sys/uvm/uvm_fault.c:1025
#20 0xc04b3ef9 in trap (frame=0xce12c974)
     at /usr/src/sys/arch/i386/i386/trap.c:631
#21 0xc010c0b6 in calltrap ()
#22 0xc040d116 in uiomove (buf=0xcdb3c000, n=8192, uio=0xce12cb90)
     at /usr/src/sys/kern/kern_subr.c:178
#23 0xc0305451 in msdosfs_read (v=0xce12cb08)
     at /usr/src/sys/fs/msdosfs/msdosfs_vnops.c:498
#24 0xc0456ac7 in VOP_READ (vp=0xd0468db4, uio=0xce12cb90, ioflag=0,
     cred=0xcebfd3e4) at /usr/src/sys/kern/vnode_if.c:424
#25 0xc0454777 in vn_read (fp=0xcebfe2a8, offset=0xcebfe2d4,  
uio=0xce12cb90,
     cred=0xcebfd3e4, flags=1) at /usr/src/sys/kern/vfs_vnops.c:474
#26 0xc0424884 in dofileread (l=0xcd54d7a8, fd=3, fp=0xcebfe2a8,
     buf=0x805e000, nbyte=262144, offset=0xcebfe2d4, flags=1,  
retval=0xce12cc68)
     at /usr/src/sys/kern/sys_generic.c:153
#27 0xc0424a3e in sys_read (l=0xcd54d7a8, v=0xce12cc48,  
retval=0xce12cc68)
     at /usr/src/sys/kern/sys_generic.c:103
#28 0xc04b3751 in syscall_plain (frame=0xce12cc88)
     at /usr/src/sys/arch/i386/i386/syscall.c:144