During the first major survival scene, you board a platform elevator that lowers slowly while headcrabs assault you.
How-To: Right after pressing the button jump off the elevator and let it descend without you, after about 10 seconds slowly slide back down to the platform, you will notice that headcrabs are no longer spawning.
Effect: no Headcrabs will not spawn
Settings: Hard difficulty
Testing: I was able to repeat the glitch 3 times, easily repeatable