Crow Chase mini-game:
Successfully complete the single player mode.
Glitch: Pet Manhack:
Take out the gravity gun, then grab a Manhack with it. While still holding it, switch to the pistol. Release [Shoot], then select the crowbar. If done correctly,the game will think you are still using the gravity gun, but you will be able to choose any weapon except for the gravity gun. If you select the gravity gun, the glitch will end.