We have a system which consists of 4 single subsystems. So I want to monitor each subsystem as well as the whole system. For each subsystem I created a host and everything is working fine. For monitoring the whole system I have to compare the sum of each subsystem's item value.
So I created a host "mainhost" having no items. Then I tried to create a trigger with the following trigger expression:

({host1:item-a.last()}+{host2:item-a.last()}+{host3:item-a.last()}+{host4:item-a.last()})>600

After clicking on the "Add" button the trigger was added for the 4 subsystems and not for the main system. This behaviour is reproducible and not a one time fault.
Additionally this is the same use case as described in example 13 in the manual. So i think this is a bug in the Zabbix frontend.