I keep having this vision that I’m watching a movie in which a couple Black Mesa scientists have another scientist (or perhaps guard) on an operating table with a headcrab on his/her head. They are reporting that the headcrab is pumping something into the stomach. They cut open the shirt and see yellowish fluorescent lines blooming across the chest and stomach illustrating that bloodstream is filling with headcrab juice.
While they’re remarking on what they’re seeing, the torso suddenly and violently splits open and the now headcrab zombie begins screaming, gets up, and throws a scientist against a far wall. As the other scientists are cowering in a corner, there is an explosion and a dead headcrab flies into the operating room.
The camera cuts to a closeup of the headcrab zombie’s hand as it begins to transmogrify into very long claws. Gordon pulls out his SPAS-12, cocks it, the scientists are bug-eyed, and Gordon blows the zombie away.
The entire scene plays out vividly in my head whenever I think of a Half-Life movie.