An unused enemy named "Mr. Friendly" can be found in the Half-Life's data. According to the book "Half-Life 2: Raising the Bar" Mr. Friendly's attacks included knocking Gordon's weapons out of his hand, knocking off Gordon's glasses (making the screen go blurry) and even raping the player. It was created by Ted Backman, and the design was because Ted "decided to go with something that elicits a homophobic response". The in-game texture is also signed by Ted.

Gordon Freeman's original design was nicknamed Ivan the Space Biker, his beard and futuristic suit are what got him this name.

In Half-Life and Half-Life: Blue Shift, one of the stock quotes from various security guards is: "Catch me later and I'll buy you a beer." When Gordon meets Barney Calhoun (an ex-guard from Black Mesa) in Half-Life 2, Barney says, "Now...about that beer I owed ya."

In original copies of Half-Life, During Chapter 3, there is a secret room in map c1a1c that can only be accessed by enabling cheat mode and noclipping through the floor. The room is made of tiled pictures of Valve co-founder Gabe Newell. The room still exists in later versions of the game, although the imagery of Newell has been removed and attempting to enter the room takes you back into the normal map.

There was a Dreamcast port in development at the same time as the PlayStation 2 version. The port was being developed by Gearbox Software and was going have the Blue Shift expansion included. Though is was almost finished and was pretty much ready to ship, the port was cancelled "due to changing market conditions."

The premise for this game was inspired by the Steven King book, The Mist, which is about how a slew of monsters ravages a city after a test in a secret military base goes awry. The code name of the project, Quiver, is a reference to the name of the military base, Arrowhead.

The Gluon Gun's entity name is "weapon_egon". This is a reference to the character Egon Spengler from the movie Ghostbusters. The weapon also resembles a Proton Pack from the Ghostbusters movie.

In the chapter "Forget About Freeman", using the "noclip" command from the game's console allows the player to enter an elevator, revealing a red triangle. Interacting with this triangle will either crash the game, or send the player two rooms ahead, dead on the ceiling.

The German version of Half-Life was censored to avoid a USK 18 rating. All the human soldiers were replaced with robots, and when a scientist or security guard is shot they just sit on the ground. Blood splatters from humans were colored yellow, although the blood textures on the environment were not changed, and the aliens still have red blood. Gina's, Gordon's, and the scientist's models were replaced with a helmeted version of Gordon's model to make them look less human. Half-Life's entry in the German index of censored games was eventually removed in April 2017, allowing Valve to distribute the game in Germany in its original state.

There is an unused NPC behavioral script for the Hound Eye, which causes it to approach machines or blinking lights and paw at them out of curiosity.

The Gonarch was born when one of the developers asked, "Why don't we put a giant testicle on a 20-foot-tall armored spider?"

According to the Valve Timeline on the Valve Employee Handbook, after internal reviews during 1997, Half Life was deemed not good enough to ship and the "Half-Life team returns to the drawing board and essentially starts over."

The design of the "Polyrobo", a robot used to test how well the Half-Life engine could take high polygon, bone-based animations, is based on the Regult BattlePod from the anime Super Dimension Fortress Macross (known as Robotech outside of Japan).

The damage done by the Houndeye pack's sonic attack increases exponentially, depending on the number of Houndeyes in the pack. If a squad leader is killed, the entire squad is weakened. Their shock wave colors range from weakest to strongest as white, sky blue, dark blue, and violet.

The lines of the HECU grunts and the VOX announcement system were made using individual word sound files that play in sequence to form sentences, the lines for the grunts having a normal and a screaming variation.

The AI has the ability to react to certain smells, such as the guards commenting on the scent of corpses or a Bullsquid detecting nearby food by scent. Additionally, there's a removed feature of the scientist commenting on smell, much like the guards.

There are two unused NPC behavioral scripts for the Vortigaunt, one which causes it to flee the battlefield if if sees the corpses of other vortigaunts and another one where a vortigaunt could resurrect a teanmate of it's kind if they witnessed it's dead.

While in an interview with Valve News Network in February of 2018, Gray Newell (the son of Gabe Newell) revealed that the final boss of Half-Life, the Nihilanth, represented him as a baby, stating: "The Final Boss of Half-Life 1 actually the 'giant evil baby monster' is supposed to be me. When my mother was in childbirth they were thinking 'what's something really scary,' and at the time, having a child seemed to be the most scary thing to everyone."

When a human character is gibbed, there is a 5% chance that the human skull will fly directly at the player's face regardless of the player's distance from the target.

After getting the HEV Suit at the start of the game, Gordon Freeman can open his work locker nearby and collect a battery to power up the suit. A picture of a newborn baby can also be found in the locker, but contrary to popular belief, this is not his child. The game's manual includes Freeman's Black Mesa job acceptance letter, which reveals that he is "unmarried and without dependents". The real identity of the baby is Isabel Teasley, the daughter of Half-Life artists Harry Teasley and Yatsze Mark born in January 1998. Teasley originally hid the picture on a broken desk in a ruined office on a completely different map as a personal Easter egg. However, level designer John Guthrie discovered this, and decided to replace a pin-up poster Teasley put in the locker with the picture. Teasley recalled the birth of his daughter as a very difficult time, as she was born with a developmental disability and needed to be fed through a tube in her stomach every two hours, while the last year of Half-Life's development when she was born was dominated by crunch work. Series writer Marc Laidlaw stated in an email to a fan in 2006 that the writing team had no reason to elaborate on who the child is supposed to be in-game, but because the intention of putting the picture in the locker was to show something that had personal significance to Freeman, he suggested she could probably be his niece.