Creepy Cortana Glitch
From Halopedia, the Halo wiki
The Creepy Cortana is a glitch that only occurs in the Halo: The Master Chief Collection version of Halo 2 after the final cutscene on the last level. It can only be viewed in the classic mode of the Master Chief Collection. This particular glitch which was introduced in the Master Chief Collection, causes a very disturbing image of Cortana to appear in which she lacks half of her face textures revealing a floating eyeball as well. While she does appear to be motionless, she will occasionally blink once in awhile.
To experience this glitch, the final level of Halo 2, The Great Journey, must be completed in re-mastered or anniversary mode. The final cutscene must be viewed to the very end. After the Gravemind talks to Cortana and she responds to him, when the screen goes black, switch to classic mode and Cortana will have half of her face missing. On some occasions, when playing the Final Four cross-game playlist, the level will be unable to progress to the Halo 3 level and will be stuck on a black screen if the player remains in re-mastered mode or will be stuck on this glitched version of Cortana in classic mode indefinitely.