Hidden subtitle in Halo 2
From Halopedia, the Halo wiki
In the level's opening scene, just before Rtas 'Vadumee ("Half-Jaw") says "And continue our march to glorious salvation!", another subtitle will appear for a split second. Right when the Elites say "We shall grind them into dust.", the subtitle will say "- scrape them as excrement from our boots!"
Please note that you must have the subtitles in the options menu set to the on position or have the Black Eye Skull activated.