Deus Ex: Human Revolution, containing a rough version of the early sections of the upcoming game, and leaking it onto BitTorrent sites at the end of May.
Details of how the build was stolen have been revealed in the court documents. When demoing the game for an Italian games magazine GMC, the thieves found the preview build on the magazine's servers, where they copied it and subsequently uploaded it. The magazine has not been implicated, since the intrusion was from an IP unassociated with the magazine, nor has the reviewer who was given access to the files. Regardless, GMC's lack of security allowed the build to be swiped.
Square-Enix is claiming the theft constitutes copyright infringement, as well as claiming damages ("in excess of $5000").