In the world of Paranoia (the RPG of the darkly humorous future), a "Code 7" refers to any mission so difficult, so dangerous, that successful completion will require that each Troubleshooter involved use all seven of their six assigned clones. (This is, of course, impossible.)
Suggestion that The Computer would ever assign its loyal Troubleshooters with an impossible mission is treason, and rumours to this effect are the obvious work of commie mutant traitors. (For more information on this definition of Code 7, readers are directed to just about every Paranoia adventure ever published.)
In GM-speak, a Code 7 adventure can also refer to any short mission ideas that were never developed into a full adventure, yet were published anyways. (See the section labeled Code 7 in Acute Paranoia, West End Games, 1986, for several examples of these.) However, given the ratio of Paranoia-GMs to Troubleshooters, this definition is not as widely used.
As always, the above information was classified Ultraviolet. If you do not have Ultraviolet security clearance, then please remain where you are and a Vulture Squad will be along shortly to assist you with termination. Have a nice daycycle, and remember: The Computer is your friend.