GTA 6, Half-Life 2, The Witcher 3, and other games sprung huge leaks before they launched.
Just recently Rockstar suffered a "network intrusion" that lead to around 90 videos of GTA 6 footage being leaked to the internet. The FBI is already investigating.
Prior to release, a massive Google Drive info leak spilled the beans on the RPG's story, enemies, quests, notes on "boob physics," and even the game's multiple endings.
Details of the upcoming Mirage were leaked ahead of this year's Ubisoft Forward presentation. Before that, lots of AC Syndicate info was leaked well in advance of that game's launch years ago.
After an early build of Mass Effect 3 somehow leaked onto Xbox Live, dataminers dove into the code and resurfaced with a copy of the unfinished script.
We learned of the existence of Dishonored 2 in an embarrassing way: Bethesda broadcast an E3 rehearsal of the game's announcement live on Twitch. D'oh!
In 2003, a pre-alpha, entirely playable version of Stalker was leaked onto P2P networks. The final game wouldn't come out for another four years.
Valve's network was cracked and the source code, maps, and playable builds of HL2 were stolen and leaked. It ended up delaying Gordon's landmark sequel by 13 months.