Before Gordon Freeman was relieved from his suspension stasis before the beginning of Half-Life 2, Alyx Vance went off to guide citizens and refugees to Black Mesa East, and Kleiner and Barney were stuck in Kleiner's lab building the older model of the Kalabi Yau teleportation device you see in the second chapter of the game.

They needed something to test the teleportation mechanism of the machine without using a human being, Kleiner refused to use his pet head-crab Lamarr so instead, they found a stray cat in the open of City 17. Barney proposed the use of the cat and Kleiner agreed.

They both ran a test using the previousely mentioned cat and unfortunately, when Kleiner teleported the cat back, it came back inside-out and later exploded in front of both Barney and Kleiner, suprizingly giving Barney nightmares for weeks on end. Even now Barney still mentions the cat as he doesn't feel comfortable and believes because of what he did, the cat haunts him since then. This was put into the game as an in-joke by Valve; Barney and Kleiner swore never to tell anyone about it until it is accidentally mentioned by Barney in the second chapter of Half-Life 2 leaving Alyx in a stream of unanswered questions throughout the chapter's cinematic. Barney also says to Gordon in the chapter "Anticitizen One", "Did you hear a cat just now? That damn thing haunts me."

Barney is now known for having a great fear of felines and may be easily prone to Post Traumatic Stress Disorder as he has the same fear of Kleiner's pet headcrab Lamarr and other head crabs. Hence why he refers to them as "headhumpers" instead of their colloquial given name.