That takes me back. I remember once, when the Aliens Total Conversion came out for Doom (the +original+ doom), everyone in the office stayed late to play a networked game.
We got down to about level 3 (the top levels were basically all empty, just like the movie, lots of walking around). We got into one long corridor and you could hear the scratching around but couldn't see anything.
Then, all of a sudden, aliens came literally out of the walls (I learned later the level designer had used Doom's one way walls and drop down doors to a VERY DRAMATIC effect here)!
Two of us (myself included) jumped so hard we fell out of our chairs. Banged my head pretty good on a desk too.
Good times