I have this trigger for "host unreachable" alerts.

{Template_Linux:agent.ping.max(300)}

=0 |

{Template_Linux:agent.ping.nodata(300)}

When agent is down, it says "Evaluation failed for function agent.ping.max" (and the state is OK). But it doesn't matter what agent.ping.max returns if agent.ping.nodata returns 1. The result will always be 1 in that case.