Final Grunt

The Final Grunt is found cowering on the edge of a cliff right before the Forward Unto Dawn section on the final level in Halo 3. If the player approaches him, he will taunt the Master Chief about his past, then offer to clean his boots and helmet if he is rescued.

However, discovering the "Final Grunt" leaves the player unable to complete the end of the level (and the game), because there will be no route to the end since the platforms will have fallen.