Impossible things, and things that just shouldn't be...

[Dario D.]

Nah the G-Man has an 'arrangement' with the zombies.

That scene makes him seem really mysterious though... The way he so casually walks into the red barn, completely uncaring, and you arrive and find zombies.

But honestly, that probably wasn't really the G-Man... it was probably his stunt double (notice he's conveniently too far away to tell), so he walks into the barn, gets headcrabbed, goes to head-crab university (as mentioned earlier), gets a PhD in chemistry, changes into a white labcoat like all the other head-crab doctorates, then attacks you as you arrive.

Valve is too lazy to hire a stunt man AND another zombie.