It's pretty obvious that the lambda symbol was chosen for the Half-Life logo as lambda is used in Real Life to represent the decay constant of a radioactive substance, which is closely related to its half-life. Less obvious is that the lambda symbol looks like an arm holding a crowbar.
Barnacles can be killed by the Crowbar in one hit. Barnacles tend to be scraped off of ships with objects like, you guessed it, a Crowbar.
The G-Man is responsible for all of the Benevolent Architecture that we've learned to take for granted in games; every single door you need to go through being conveniently unlocked, for instance.
Why did the HECU go in to silence everyone in Black Mesa? If they rescued everyone, the Combine would have had far more brains to suck out for knowledge on teleportation technology.
The first time you realize that headcrab zombies are most likely semi-aware, either from the Zombines calling for help over their radios or accidentally happening across the reversed zombie audio on Youtube.
Gordon Freeman is never seen wearing a helmet, making his resistance to Headcrabs, Gunshots and Explosions quite strange.
Better yet, the PCV, which protects you from all the same things the HEV suit does but only covers the torso, sure the shield covers the whole body (Just like the HEV), but still.
Shephard averts this. In addition to wearing the PCV vest, he also wears high grade military boots, a sturdy combat helmet and a gas mask. Considering all this, one might say that he's better off than Freeman. ~The-Artist-64