An immigration agent in Shimonoseki asked me to prove that I had sufficient funds to support myself in Japan because I hadn't booked a ticket out of Japan. The circumstances were a little special - I was already in Japan, and had just changed my status from a working visa to a tourist visa. It would have been nice if he'd told me that he was going to ask for proof of onward travel before he rescinded my working visa.

I have always assumed that the immigration agents at airports have some data about the onward travel arrangements of arriving passengers, but perhaps I am wrong about that.