Some PC gamers reported on forums like Reddit, Steam and the Call of Duty forum that their PC copy of Call of Duty: Black Ops 2 second disc was Mass Effect 2.

The cause of the error is unknown, but it’s clear that something went wrong along the production line.  Affected gamers can enter the serial number on the box into Steam’s product activation window to activate the game. However, they will have to wait a few hours for the download to finish.

Source: PCgamer