[root@zabbix sbin]# zabbix_get -s localhost -k proc.num[zabbix_server_mysql]
0
[root@zabbix sbin]# zabbix_agentd -V
Zabbix Agent (daemon) v1.8.11 (revision 26205) (20 March 2012)
Compilation time: Mar 26 2012 04:14:38

[root@zabbix sbin]# ps ax | grep -c zabbix_server_mysql
29

I stopped new agent and replaced zabbix_agentd with old binary and started old agent:

[root@zabbix sbin]# zabbix_agentd -V
Zabbix Agent (daemon) v1.8.10 (revision 24303) (28 December 2011)
Compilation time: Dec 29 2011 06:15:20

[root@zabbix sbin]# zabbix_get -s localhost -k proc.num[zabbix_server_mysql]
27