Share 'what not to do in Japan?'

i've just about 2 weeks left to prepare to spend 4 months in Japan. my first trip (several years ago) passed without my cheeks ever recovering from perpetual blushing. short of walking into my friend's house with my shoes on and sticking my chopsticks into my rice bowl, i'm sure to have committed every faux pas imaginable. those i was visiting were too kind to tell me about most of it... so he…