Prior to using the net.dns item key, all host resolutions (using web.page.get item key) use the name servers configured in /etc/resolv.conf
After using the net.dns item key, some of the host resolutions use the name server provided in the net.dns item key above.

This caused some web.page.get to fail, as it cannot resolve the host name.

/etc/resolv.conf:
nameserver 192.168.0.4
nameserver 192.168.0.5

name server used with net.dns
zabbix_get -s127.0.0.1 -p10050 -k net.dns[69.164.211.158,pagekite.com]

web.page.get
zabbix_get -s127.0.0.1 -p10050 -k web.page.get[www.google.com]

A restart of the Zabbix agent is needed in order to remove this "cached" name server entry.