I was messing around with dashboard filtering in Zabbix 2.2.1 and found that it will incorrectly hide hosts. Here's how to replicate the issue:

1. Create a new host called "test1" and add it to hostgroups called "testgroup1" and testgroup2".
2. Create a new host called "test2" and add it to the hostgroup called "testgroup2".
3. On the dashboard, notice that 2 hosts are listed in the "Host Status" widget for the hostgroup "testgroup2".
3. In dashboard filtering, enable filtering for selected hostgroups.
4. Exclude "testgroup1" and save the filters.
5. Notice that the "Host Status" widget now only shows one host for the hostgroup "testgroup2".

The results of this are incorrect or the filtering options are incredibly misleading. I only hid the hostgroup "testgroup1", not all the hosts in hostgroup "testgroup1". Two hosts should still show up in the "Host Status" widget for "testgroup2".