Active only clients (behind NAT) are usually configured with agent interfaces 0.0.0.0 IP. Zabbix server should be smart to recognise 0.0.0.0 as special configuration and avoid connecting to that IP.

If server connects to that ip, it will connect to its own agent. Because hostnames are not checked for outgoing connection, it will assume that he is connected to right client and instead of monitoring client, it will monitor self. This happens if some items on NAT monitored servers are not marked Agent active.