OS X Mavericks is compatible with most Macs that are capable of running OS X Mountain Lion; as with Mountain Lion, 2 GB of RAM, 8 GB of available storage, and OS X 10.6.8 (Snow Leopard) or later are required.
The full list of compatible models:
mid-2007 or newer iMac
aluminum MacBook, 2009 or newer polycarbonate MacBook
2007 or newer MacBook Pro
late-2008 or newer MacBook Air
2009 or newer Mac Mini
2008 or newer Mac Pro
So, take a mac pro earlier then 2008 and install mavericks on it. Cant? Sure you can, you just have to "hack" the installer.
So like i said, arbitrary limits.... The box can run it, just it wont install without modifying the setup program.