In Korea, it is generally believed that leaving a fan on in an enclosed room can be fatal. Ken Jennings looks at cultural superstitions and wrote a Slate article focusing on the scourge of Korean fan death.
My mother told me that handling cellophane tape makes you sterile. Though that may have just been her way of getting me to stop playing with cellophane tape.
What strange cultural superstitions exist in your part of the world? (Of course, this is a bit of an unfair question, because if you genuinely believe it, then you won't recognize it as a strange cultural superstition!)
Clarification: Please reply in the spirit of the article. Keep it fun.