If, say -- and please note this is purely theoretical! -- if you were to realize half way through your flight from the US to Tokyo that you'd grabbed the wrong passport and the one with you was expired (but you've got a second, still valid passport back at home), what would happen when you landed in Japan?
Say, too, that you've got scans of your valid passport and have traveled on it several times this year to Japan. And you've managed to get your valid passport sent overnight to arrive the next day. Any chance you aren't immediately sent back?