With a zpool based on two of the geli vdevs
configured with the /etc/rc.conf lines:

   geli_devices="ad0s1 ad0s3f ad0s2"
   geli_ad0s1_flags="-k /root/ad0s1.key"
   geli_ad0s2_flags="-k /root/ad0s2.key"
   geli_ad0s3f_flags="-k /root/ad0s3f.key"

zpool scrub causes the following panic:

Unread portion of the kernel message buffer:
GEOM_ELI: Detached ad0s2.eli on last close.
GEOM_LABEL: Label for provider ad0s2 is msdosfs/ÒA.Û,{(#0.
panic: Function g_eli_orphan_spoil_assert() called for ad0s3f.eli.
KDB: enter: panic
panic: from debugger
Uptime: 5m27s
Physical memory: 1014 MB
Dumping 120 MB: 105 89 73 57 41 25 9

#0  doadump () at pcpu.h:195
195     pcpu.h: No such file or directory.
        in pcpu.h
(kgdb) bt   
#0  doadump () at pcpu.h:195
#1  0xc05db8f3 in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:409
#2  0xc05dbb1c in panic (fmt=Variable "fmt" is not available.
) at /usr/src/sys/kern/kern_shutdown.c:563
#3  0xc04a40c7 in db_panic (addr=Could not find the frame base for "db_panic".
) at /usr/src/sys/ddb/db_command.c:433
#4  0xc04a4825 in db_command_loop () at /usr/src/sys/ddb/db_command.c:401
#5  0xc04a6255 in db_trap (type=3, code=0) at /usr/src/sys/ddb/db_main.c:222
#6  0xc06014a4 in kdb_trap (type=3, code=0, tf=0xf4bcac1c) at /usr/src/sys/kern/subr_kdb.c:502
#7  0xc0838d2b in trap (frame=0xf4bcac1c) at /usr/src/sys/i386/i386/trap.c:621
#8  0xc082120b in calltrap () at /usr/src/sys/i386/i386/exception.s:139
#9  0xc0601602 in kdb_enter (msg=0xc0889693 "panic") at cpufunc.h:60
#10 0xc05dbb05 in panic (fmt=0xc3e9a0e8 "Function %s() called for %s.") at /usr/src/sys/kern/kern_shutdown.c:547
#11 0xc3e92d65 in ?? ()
#12 0xc3e9a0e8 in ?? ()
#13 0xc3e99d84 in ?? ()
#14 0xc3e0d890 in ?? ()
#15 0xf4bcacac in ?? ()
#16 0xc058eff5 in g_spoil_event (arg=0xc3c92940, flag=-945983104) at /usr/src/sys/geom/geom_subr.c:903
(kgdb) f 16
#16 0xc058eff5 in g_spoil_event (arg=0xc3c92940, flag=-945983104) at /usr/src/sys/geom/geom_subr.c:903
903                     cp->geom->spoiled(cp);
(kgdb) l
898                     if (!cp->spoiled)
899                             continue;
900                     cp->spoiled = 0;
901                     if (cp->geom->spoiled == NULL)
902                             continue;
903                     cp->geom->spoiled(cp);
904                     g_topology_assert();
905             }
906     }
907

Problem first reported in:
http://lists.freebsd.org/pipermail/freebsd-current/2007-October/078105.html

Fix: 

Quoting Pawel Jakub Dawidek's response to my initial report:

|GELI's detach-on-last-close mechanism is a general purpose mechanism, it
|may not work correctly with ZFS, because ZFS sometimes closes and reopen
|providers, which will make GELI to detach. In other words you shouldn't
|configure detach-on-last-close for ZFS components. It shouldn't panic
|still.

Adding geli_autodetach="NO" to /etc/rc.conf indeed prevents the panic.

I previously wasn't aware that this option exists,
so maybe it should be mentioned in geli(8).
How-To-Repeat: Create a zpool with more than one geli vdev
that detaches on last close and run zpool scrub.

Actually I am not sure if a zpool with only one
such vdev is guaranteed to work, but at least
for me the problem only started to show when I
added the second one.