cpio reports as an error an attempt to archive a named socket:
$ pwd
/tmp
$ find . -print|cpio -ocv > /dev/rmt/0
.
ps_data
.rpc_door
.rpc_door/.rpc_100029.1
.rpc_door/.rpc_100029.2
.rpc_door/.rpc_100029.3
.pcmcia
.pcmcia/pcram
.X11-unix
cpio: ".X11-unix/X0" ?
.X11-pipe
.X11-pipe/X0
.removable
.regtool
cpio: "blah-14476232" ?
.mnttab.lock
.name_service_door
.obp_devices
.pwd.lock
.reconfigure_power
.rootkey
.sysIDtool.state
.sysidconfig.apps
.syslog_door
.login
31 blocks
2 error(s)
$ echo $?
2
$ file .X11-unix/X0
.X11-unix/X0: socket
$ file blah-14476232
blah-14476232: socket
This is not only reported on the cpio listing output, but also reflected in the cpio exit code. Backup scripts which are aware of this code will see this as a failure to back up.
Work Around
add a check for sockets to backup scripts where possible:
from find . -print|cpio -ocv > archive
to find . ! -type s -print|cpio -ocv > archive