Portal spoilers - which you don't want to read if you've never played because going into the games unaware is unbridled joy.
Spoiler warning last chance!
The most-back-stabbey-and-I-feel-personally-and-intimately-betrayed video game antagonist Wheatley has put you through a series of traditional test chambers, all the while Potato!GLaDOS is quipping along. With Wheatley goading about your imminent demise a few chambers ahead, GLaDOS assures you she can free you both soon.
But surprise, Wheatley springs his trap early and you are catapulted off into another section of the Enrichment Centre and on to a platform surrounded by Crushers - a moment GLaDOS announces as "the part where he kills us!".
To which Wheatley joyously declares this is "the part where I kill you".
On screen: Chapter 9: The Part Where He Kills You
Achievement Unlocked: The Part Where He Kills You.
Of course these are all clearly on-screen, so the Easter Egg is the additional mention of the phrase - it's the name of the track being played!
And as one final Egg, when you escape, Wheatley begs you to come back.
And you can - dumbfounding Wheatley (and GLaDOS), who did not plan you would actually return to the trap he can no longer. What follows is a two minute of further dialogue that most players would never hear in their adrenaline-filled flight. Wheatley makes a list of unbelievable promises (from Chell's parents to a newly designed fashion jumpsuit from France). Just before you do or do not jump in, GLaDOS makes one final remark.
"You really do have brain damage, don't you?"