Hello everyone, I made a video where I was playing with the physics of this game, I suppose that in newer versions things like this will be fixed, but sincerely, don’t do it!! 
https://www.youtube.com/watch?v=8g_QBBKF1t0
Hope you enjoy, and thanks for this wonderful mod!
Well, technically, the NPC’s head is a box for the physics engine. That’s just how Source physics work. You can also stand on it.
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.