Eating peanuts will make your nose bleed.
Leaving a window open while you sleep will give you a stomach ache.
Leaving a fan on while you sleep will make you sick and may kill you.
Drinking soda (especially dark ones) will melt your bones.
Leaving a bottle of water outside your home will keep cats away.

Can anyone think of others? I'm not sure how many people believe these, but I've heard and seen all of these in Japan (some several times). The fan one is really strange. I remember a few times in the summer when I spent the night with my girlfriend and she would turn the fan off in the middle of the night because she was concerned. I nearly died from heat stroke! Anyone who knows how miserable a Japanese summer is can attest to that.