I know the roof of Mercy Hospital better than you could possibly imagine.
In my mind is a very detailed map of that place, all its topography laid out clearly. I could draw you an overview from memory.
The metal walkway leading up to the heli pad.
The narrow stairway that leads up to the air conditioning unit platform.
The two catwalks that flank the mounted machine gun.
Oh, hello! I'm talking about video games!
Of the four extraction points in Left 4 Dead's zombie apocalypse simulation, the roof of Mercy is widely regarded as the most difficult. It's got almost no open ground, so very little room to run. You constantly have to be aware of what's behind you so you don't get pinned in a corner. Compared to the other extraction areas (an airport runway, a farmhouse in an open field, and a lakeside camping area) that roof is like a postage stamp. A postage stamp of a middle finger.
And that would be enough by itself, but the icing on the difficulty cake is that this is all on top of a very large building. So if you get hit by a tank in the wrong spot, you go flying right the hell off the side.
That ladies and gentlemen, is not a "game mechanic." That is a "big 'screw you' from the programmers."
But Mercy is possible, even on Advanced mode where the proverbial men and boys are sorted into their appropriate groups. I've done it. We've done it (the we being my usual crew.)
But not without a lot of trying that level, over and over again.
I am still on the roof of Mercy Hospital being decimated. And I am on that helicopter with all 3 of my friends, looking back at the place that tested us. And I am standing in that chopper watching our fourth man get dragged away before anyone could help him. And I'm standing there alone because I left them all.*
I had to.
*And then we watched and episode of "Man v. Food" and called it a night.