Medical myths intrigue me. I think its partly because they're often a combination of public ignorance, mixed with a little ignorance on the part of medical professionals.
The British Medical Journal last issue of the year lists recently debunked medical myths. Last year, they highlighted the myth, which doctors keep repeating, that you need to drink 8 glasses of water a day. Comes to find out that number was basicaly made up. In any case, our food has a lot of water in it, so thinking about by glass is meaningless. You should drink water when your body wants it! Other good myths include reading in the dark is bad for your eyes and mobile phones are bad for hospital equipment (actually, giving doctors mobiles phones in the emergency room is a good idea as it increases communication)
This year, BMJ has a new list, and its just as interesting. Most important for me, eating before bed does not cause you to gain weight! Also, sugar does not cause hyperactivity in kids, suicides do not increase during the holidays and poinsettias are not dangerous to eat.