Sometimes things that seem completely harmless just come and hit you over the head…
That’s what happened to me with Zabbix yesterday. I was going through some items and cleaning up, when suddenly, 4 hosts went off-line. Except that they hadn’t.
I couldn’t understand why Zabbix refused to see them (the agents were responding fine).
I finally found out that it was because of the way that Zabbix calculates the ’status’ item, which has a value of 0 if alls well, and 2 if the host is off-line.
Zabbix looks at the first item to be taken of the host, if that item returns an empty string, Zabbix decides that the host is dead and raises the alarm, even if the host has 150 other items all working perfectly. This bizzar to say the least, but workable. If this happens, a quick fix is to deactivate the offending Item, so that the first Item checked returns a value, then the host comes back 
Any how, I thought I’d just document this in case anyone else starts loosing hair over the issue