Users on Reddit, Steam and the official Call of Duty forums have discovered that the second disc is actually EA's space franchise.
While affected consumers won't be able to install the game from the disc, it is possible to register the serial code into distribution service Steam to download the remaining content.
Believed to be a production issue, Activision has yet to provide a comment on the mistake.
Released worldwide today (November 13), a number of UK retail offers see the first-person shooter heavily discounted.
Copies of the game were released early by certain retailers, which will lead to publisher Activision taking action at a later date.
Black Ops 2 will be available on Xbox 360, PS3 and PC tomorrow (November 13) worldwide, and on Wii U from November 30 in Europe.
> Launch guide: Everything you need to know about Black Ops 2
> Black Ops 2 review round-up: Critics score the latest Call of Duty
Watch a trailer for Black Ops 2 below: