Hit game Minecraft's third-party server software, CraftBukkit, was recently hit by a DMCA notice from none other than one of their developers, Wesley Wolfe, also know by the handle Wolvereness. This DMCA notice states that CraftBukkit is infringing on Mojang's code.
The provided license requires the use of included or linking code to provide the original source under the GNU GPL license version 3, or any later version. An official notice has been sent to Mojang AB, whereas the Chief Operating Officer, Vu Bui, responded with the clear text:
Mojang has not authorized the inclusion of any of its proprietary Minecraft software (including its Minecraft Server software) within the Bukkit project to be included in or made subject to any GPL or LGPL license, or indeed any other open source license.
This issue primarily stems from the fact that the Bukkit project is licensed under GPL. One of the clauses of this license states that all code in an GPL project must be under this license. Technically, the Bukkit project is indeed in infringement of this, since the vanilla Minecraft server software (which CraftBukkit uses as a base) is not licensed under GPL. Instead, it uses licensing from Mojang's own EULA.
Because of this sticky situation, given that many third party server softwares for Minecraft use the vanilla Minecraft server software as a base, many other projects and forks of CraftBukkit can be a victim of this DMCA claim; in fact, an optimized build of CraftBukkit, Spigot, has already had a DMCA complaint filed against them by Wesley Wolfe, under similar reasoning.
Currently, Mojang has failed to come forward with an official response as to what exactly they'll do about this, although Mojang's Chief Operating Officer, Vu Bui, acknowledges the issue, letting us know that talk of this is likely circulating Mojang right now; we'll hopefully see a response soon. In the meantime, a multitude of pages are unavailable, including CraftBukkit's download page. Spigot's download page still remains accessible, however.