I've yet to beat HL2 and I've had it the day it came out. I think I stopped just after the citizens rebel. Usually, I'm screwing around with the game files trying to make it more to my liking. I always hated how the shotgun only carried six shells in its magazine and I could only carry 12 .357 Magnum slugs at any one time.
The thing about having to travel from Dr. Kleiner's lab to Black Mesa East was that during the malfunctioning teleportation, you teleport into Dr. Breen's office. He knows exactly who you are and what you're capable of, so he has to warn his Combine overlords and send out all those scanners to find you. Dr. Kleiner and Barney couldn't let you back in for fear that your presence will bring the Combine down on them and risk everything they've worked for the past few years.
Besides, if you had teleported directly to Black Mesa East, you wouldn't get the chance to shove the Combine machine gun right up that hunter-chopper's exhaust port!
