Sentinels

The Sentinels were a race of robots built by the Janitor to help him clean up the ridiculous amount of dirt that had built up on the Halo Rings. They were armed with lasers that fired burning Cillit Bang at dirt and mess, designed to lift out stains and clean even the toughest of grime buildups.

Unfortunatly for the Galaxy, the idiots at the Forerunner Council thought that the Sentinels were a battle robot and replaced the Prometheans, a highly advanced fighting machine, with cleaning droids. Their weapons proved inneffective against the Flood, no surprises there.

The only thing that the Sentinels did of any use during the Human-Covenant war was take shots for their allies. For example, during the battle of the Ark, Guilty Guilty summoned hundreds of Sentinels to go to Truth's Citadel, so that the Grunts would waste all their ammo destroying them.