Half-life 2 Walkthrough -> Arrival
Pass the guard (he'll be wearing an armored suit with a mask resembling a gas mask—they're called "Combiners"; there are three types of them, more on that later), who will rather rudely kick you in the chest, through the turnstile, and to the left, into the waiting room. A man at the table will advise you not to drink the water, saying they put something nasty in it.
Then a new guard will lead you into a side room, shouting, "Come with me, citizen!" When he removes his mask, you recognize an old man named Barney (the guard from the first game), and then you'll connect via video link on a monitor with Dr. Kleiner from the same first game:
It looks like these two are up to something. It also looks like the deception has been revealed, and enraged soldiers are about to burst into the room. Without further ado, exit through the next door, up the stairs, push the crate against the wall, and jump through the window (press "E").
On the roof, run to the paint cans. Behind them, there will be planks you can use to climb up to the ledge, which will lead you to an open window. Once you've climbed through, save immediately—I'll explain why in a moment. After saving, go down the stairs. The Combine will arrive and start beating you up.
Load a save and the glitch should disappear. She takes us to Kleiner's lab, where we'll be unsuccessfully teleported later, but more on that in the next chapter, "A Great Day."