I'm getting assertions here for certain listvals.
After some investigation, it seems this occurs when there are instances differing only slightly, with a digit at the end of the string, e.g.:
<pre>
ccsvli63_root# collectdctl -s /var/run/collectd/sock show host=openafs plugin=openafs_vos plugin_instance=ccafs09.in2p3.fr type=listvol_dayUse type_instance=group.aleph.prod
send:    --> LISTVAL host="openafs" plugin="openafs_vos" plugin_instance="ccafs09.in2p3.fr" type="listvol_dayUse" type_instance="group.aleph.prod"
receive: <-- 2 Matching values
receive: <-- 1353945812.138 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.prod
receive: <-- 1353945812.140 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.prod2
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.prod"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.prod2"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
+-----------------------------------------------------------------+------------+

! Name                                                            !      Value !

+-----------------------------------------------------------------+------------+

collectdctl: collectdctl-show.c:572: write_data: Assertion `g->num < 2' failed.
! openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-gr !Aborted
</pre>

another example:

<pre>
ccsvli63_root# collectdctl -s /var/run/collectd/sock show host=openafs plugin=openafs_vos plugin_instance=ccafs09.in2p3.fr type=listvol_dayUse type_instance=group.aleph.scaleph
send:    --> LISTVAL host="openafs" plugin="openafs_vos" plugin_instance="ccafs09.in2p3.fr" type="listvol_dayUse" type_instance="group.aleph.scaleph"
receive: <-- 7 Matching values
receive: <-- 1353945812.382 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph
receive: <-- 1353945812.078 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph2
receive: <-- 1353945812.067 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph3
receive: <-- 1353945812.344 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph4
receive: <-- 1353945812.346 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph5
receive: <-- 1353945812.076 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph6
receive: <-- 1353945812.070 openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph7
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph2"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph3"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph4"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph5"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph6"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
send:    --> GETVAL "openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-group.aleph.scaleph7"
receive: <-- 1 Value found
receive: <-- value=0.000000e+00
+-----------------------------------------------------------------+------------+

! Name                                                            !      Value !

+-----------------------------------------------------------------+------------+

collectdctl: collectdctl-show.c:572: write_data: Assertion `g->num < 2' failed.
! openafs.in2p3.fr/openafs_vos-ccafs09.in2p3.fr/listvol_dayUse-gr !Aborted

</pre>