Rare added Goldeneye multiplayer at the last possible minute before launch

I will have to just borrow this poster's comments, because it sums up what I would say.

patrickadunham

They didn't start working on the multiplayer until March or April, and the game launched in August? Those were possibly the most productive 4 or 5 months in game development ever. 

http://www.joystiq.com/2012/08/14/goldeneye-007s-multiplayer-was-added-last-minute-unknown-to-ra/