The comment for the "ServerActive" parameter in zabbix_agentd.conf currently says this:

"List of comma delimited IP:port (or hostname:port) pairs of Zabbix servers for active checks."

I've noticed during the time I've spent in #zabbix on freenode that people get confused at times and will put multiple IPs in this parameter even when they only have a single Zabbix environment. It may be a good idea to mention that while it is possible to have multiple parameters in ServerActive, most cases will only want one IP.