Does anyone know origins of 万一?
http://baike.baidu.com/subview/199191/8163691.htm gives a plausible answer: it refers to a remote possibility of a thing X happening.  Sort of "among the ten thousand and one things that could happen, one is X."  Far back in ancient and classical Chinese,  the ten thousand things are a proverbial way to express all that happens in this world.  But I did not find sources giving any specific classical source for 万一 meaning "just in case."