I hate to admit that this happened to me several times!  When I do something out of habit, I don't always pay full attention to everything going on around me.  It can be easy to miss things like a rainy morning.  Of course, there were those times when I knew it was raining but I needed clean underwear so I had to wash clothes and hang them up inside.  It would be easier to live like Tanuki and run around naked but I don't think the people at work would appreciate that.  If you're living in Japan, remember to look out of the window before washing your clothes!