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 classic mode. The glitch causes half of Cortana's face textures to not appear.
Complete The Great Journey in Halo 2. The final cutscene must be viewed in re-mastered mode 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 you will be able to see Cortana with half her face missing.
When playing the mission as part of a cross-game playlist in The Master Chief Collection, the game will be unable to progress to the Halo 3 level and will be stuck indefinitely.