In bm_c1a1d, there is that huge industrial lift that you have to power on before using it to reach the bottom of a slanted pit. Normally, if you stay on the lift after having started to go down, headcrabs get out of a ventilation conduct and start to follow you in the pit.
There is a way to avoid headcrabs getting out of their conduct : right after activating the lift, run out of it, and wait a little before jumping on it again (wait for it to pass the point where is triggered the headcrabs “invasion”). There will be no headcrabs to bother you.