After the credits, wait to see the headcrab, Lamar, being pursued by the scientist. Then, watch him pounce on your head. Additionally, after you complete the game, watch the credits. Note: They cannot be skipped. When you reach the end of the credits, you will see Lamar jump onscreen. You will then hear Dr. Kliener say, "Lamar? Blast that little… Where did she get to?" This is the same thing that he says when you first see him looking for her. Lamar will then lash out at you and supposedly "couple with your head".