After a server restarts, we receive incorrect data from a user parameter check. Restarting the agent fixes the issue. This particular check grabs a cksum of the mtab file to identify volume changes.

Setting from agent config:
UserParameter=shell.check_mtab,/bin/cat /etc/mtab | /usr/bin/sort | /usr/bin/cksum | /usr/bin/cut -f 1 -d " "

After reboot on client:
sudo -u zabbix /bin/sh -c "/bin/cat /etc/mtab | /usr/bin/sort | /usr/bin/cksum | /usr/bin/cut -f 1 -d \" \""
1618330171

From server:
$ zabbix_get -s server -k shell.check_mtab
259242569

After restarting agent:
zabbix_get -s server -k shell.check_mtab
1618330171

strace with incorrect value:
http://pastebin.com/ze05tww1