Get this: Some people bought Call of Duty: Black Ops II for PC, but instead got Mass Effect 2.
Several people have reported that they bought and received two normal discs for Black Ops II, with the game logo printed on them, but when they were asked to insert disc 2 during installation, the disc turned out to be a copy of Mass Effect 2.
Reports come from Reddit, the official Call of Duty Community, Steam and YouTube as you can see above.
If you're one of the unlucky ones, you can choose to install the game via Steam and download the missing files from disc 2, but obviously this isn't exactly a great fix for people with slow internet connections.