Not really a glitch just a very amuzing and wierd thing.
In half life you pin an enemy, wait for it's body to become still ,then shoot the body again with the crossbow , but shoot it so it pins to another surface or wall, the bolt should go through the body not hit it. But if you hit it with the crowbar or shoot the body after you shoot the bolt it will start spazzin because it's trying to get to the area where it's been pinned but it's been held by another bolt already. Enjoy! :] (the farther apart the bolts are the more the body will spaz)
At the 'Selection' screen remain idle and after several minutes a demo video will start to play of one of the games in the pack. After this Demo video has finished you will be able to view the hidden Left 4 Dead trailer.
At the 'Game Select' screen where you pick one of the three games just remain idle there without selecting anything for a couple of minutes and demos of Team Fortress, Half Life 2 and Portal will start playing and after they have finished wait a couple of more minutes and a trailer for Left 4 Dead will play.