Grim Adventures Of Billy And Mandy (the) Easter Egg - Hello Halo

On the Billy and Mandy Christmas episode (the one when Santa was a vampire), watch the part where Mandy is saying Santa is an invention of big companies. While she is talking you can see a bunch of items she is thinking about. One of the items has a picture of someone who looks just like master chief and the game's title is HELLO 2.