Half-Life 2 leak

The Half-Life 2 leak, also known as the Half-Life 2 Beta or Half-Life 2 Alpha occurred on October 2nd, 2003, when a German hacker named Axel Gembe infiltrated Valve's internal network to gain access to the servers. Among the stuff he found was several in-development builds for Counter-Strike Source, Team Fortress 2 and, most infamously, Half-Life 2, which was planned for release that November.

Leaking
The source code first leaked online on October 2nd, though unlike the other leaks this one wasn't by Gembe himself; rather, it was from an individual from the hacking group myg0t who gave him the files. According to the individual who remains anonymous, he didn't give permission to Gembe to leak the build however it quickly started appearing on the internet shortly. Gembe was a fan of the original Half-Life, which could explain his motivation to leak the sequel. It didn't take long for Valve to realize that they've been hacked, as Gabe Newell explained what happened the same day.

At first, people weren't able to play the build at all due to the source code only supporting the v35/36 models, despite the models themselves using the newer v37 format. However, on October 8th, Axel released the compiled source code with support for the v37 format. Inside this in-development build, there were various maps, concepts, and characters that were removed after the game was delayed:


 * Various maps such as the Ravenholm (later very reworked and simplified), Air Exchange, Kraken Base, the Depot or Prison (later reworked and simplified as Nova Prospekt), the Antlion Caves (later recycled for Episode 2), The Borealis and various Citadel designs (one of which was going to take on an organic shape).
 * Many unused enemies, such as the returning Bullsquids (which they reduced as a mentioned enemy in the finalized version) and Houndeyes, alongside new ones such as the Combine Guard, the Hydra (seen at E3 2002), and the famous Cremator (which reduced as an easter egg).
 * There were more weapons compared to the final game. They ranged from standard ballistics such as the MP5K, AK-47, Heavy Machine Gun, and the popular OICW (seen in various screenshots and concept art) to more advanced weaponry, such as the Tau Cannon (which was originally able to be used off the jeep), the Immolator, and the Combine Guard Gun.
 * Many of the characters were very different. For example, Eli Vance was called Eli Maxwell and was a hobo scientist, Alyx Vance was not related to Eli and was the daughter of a cut character named Captain Vance There were also characters that were removed, such as Odell (who was the captain of the cut Borealis and was turned into Odessa Cubbage) Elena Mossman, who was later made into Judith Mossman, and the Combine Conscripts, the early version of the final game's rebels.

Backlash and Arrest of Gembe
The leak resulted in Valve getting a severe amount of backlash, both for allowing themselves to get compromised and for Half-Life 2's extremely buggy state, making it clear that it would miss its Christmas deadline. The backlash was severe enough for Valve to delay the game until November 16th, 2004. It's believed that the leak costed Valve an estimated $250 million.

Gembe was arrested in March 2004 after getting a fake call from FBI stating that they wanted to do a job interview, when it was really Valve, the FBI and German officers attempting to catch him. Upon arriving at the location, German officers apprehended him and he was sentenced to 4 years probation in November 2006.