Its most likely that you have. Its the time in the first level on the Pillar of Autumn, when after you get your pistol, you turn around and head for the bridge. You can shoot Captain Keyes or some of the officers and commanders there too. But shortly after you do that, Cortana makes an announcement "This is the bridge, the Master Chief has gone rampage. Marines...get him!" Then marines (who can't die) come to kill you. Its really hilarious.
Has anyone ever tried it out? I think it works on some other levels too, such as the Truth and Reconciliation one.
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