From "Apollo The Race to the Moon" pg 102:

The joke that made the rounds of NASA was that the Saturn V had a reliability rating of .9999. In the story, a group from headquarters goes down to Marshall and asks Wernher von Braun how reliable the Saturn is going to be. Von Braun turns to four of his lieutenants and asks, "Is there any reason why it won't work?" to which they answer: "Nein." "Nein." "Nein." "Nein." Von Braun then says to the men from headquarters, "Gentlemen, I have a reliability of four nines."