In the "System Status" table on the dashboard, cells can be colored red or yellow despite showing that there are 0 active triggers. It seems to do this when there is a trigger tripped that is silenced because of a dependency.

For example, in the following screenshot, the second row has two average triggers which are being suppressed because they depend on the two high triggers.

See http://lukecyca.com/tmp/zabbixbug.png