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.
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.
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.