An exchange in the HBO forum led me to try and follow the instructions to bring the Spec Ops Commander, lovingly called Half-Jaw for his mutilated mandibles, to the Control Room of Delta Halo at the end of the Halo 2 level The Great Journey. Dude, Where's My Spectre?

This one is fairly well known by now, but Jim Glover sent it to us first:

When you get the banshee to defend the Scarab, do everything as normal but where he destroys the door, break off the Banshee's wings; from there fly through the door and into the room where there are lots of brutes. Kill them, then fly full speed into the last part of the level. Once the you regain control run back to the banshee. If you flew it far enough, it should be there to use to kill Tartarus.