I had a double-cool moment happen on my first trip through. There's a spot where you come up out of the sewers and are in a building with an elevator with a bunch of barrels on it. There are also a lot of metal beams as rafters. Anyway, a Combine troop was shooting down at me and I fired a crossbow bolt at him. What I didn't notice is what it did. I get up to the top, and I see his body. The bolt went through his neck and pinned his skull to the steel girder above where he was standing. I just laughed, then other troops poured in.
His body was at just the right level so I used it for a shield. They're shooting this body up and it's just twitching and I'm leaning out from behind it taking shots back. Loved it.