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Team Fortress 2 - Did You Know Gaming? Feat. Markiplier
Portal - Did You Know Gaming? Feat. MatPat from Game Theory
 
Early on in development, the game was going to feature a "Physgun" instead of the well-known Gravity Gun, which would be able to pick up objects from far or close range, and move them around in the air. It was eventually removed because there were too many exploits it was capable of, and the concept was later re-used in Garry's Mod.
Contributed by TheProJamer
It was originally intended for the game to have 5 episodes, but only 2 were released. Episode 4 was being developed by Arkane Studios, but was cancelled due to Valve not liking the game's plot, which took place between Episode 1 and 2. Episode 5 was also being developed by Arkane Studios and was going to feature a 'Magnet Gun'.
Contributed by WaRnEvErChAnGeS
Part of the soundtrack, Hazardous Environments, is also used in the Valve intro.
Contributed by MAlpha17
When the music track Radio is played in reverse it's possible to faintly hear a woman saying either "Gordon, can you hear me? Gordon, can you hear me?" or "G-man, can you hear me? G-man, can you hear me?"
Contributed by MAlpha17
Lamarr's name is a reference to Hedy Lamarr, an actor and scientist who co-invented an early form of spread spectrum communications technology.

This is also a pun about Lamarr being a headcrab, as a play on the word "head" being similar to "hedy".
Contributed by Psychospacecow
The Ichthyosaur was to appear more prominently in Half-Life 2, such as in the the Coast levels (where it was to stop the player venturing too far into the ocean) or at the lake at the start of Ravenholm. Its only appearance in the final game was in the recycled Ravenholm lake seen during the teleportation failure sequence at the start of the game.
Contributed by RetroRex
During development, it was intended for a friendly alien creature to follow Gordon Freeman around, feasting on dead enemy bodies. It was taken out when the developers realized that the AI would be too difficult to program.
Contributed by RetroRex
The G-man can be seen in the distance at various points in the story, appearing to be watching over the player. The locations in which he appears are almost all unreachable, and he walks out of view as the player approaches.
Contributed by EFB01
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The faces of the characters in Half Life 2 are actually based on real life people. Eli's face is based on a beggar Valve found on the street.
Contributed by TheOtherWorld
Heightening the pitch of the Zombine's screams will make them intelligible. They can be heard saying "Biotics... overrun... infestation... move... target... contact!"
Contributed by xDarkAbsolute
There's an unused sound file in the game's data called "Monk_Death07". The name suggests that the developers planned to kill off Father Gregory during the ravenholm level, but the idea was later scrapped.
Contributed by Rivalremi
In the first Half-life, 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."
Contributed by Retsof-Noraa
If you reverse some of the sounds that the zombies make in the game, they can distinctly be heard yelling, "Oh God... God help me!"
Contributed by ThisIsGamer