From Wikipedia, a list of common misconceptions, including a recent favorite about life expectancy in the Middle Ages:
It is true that life expectancy in the Middle Ages and earlier was low; however, one should not infer that people usually died around the age of 30. In fact, the low life expectancy is an average very strongly influenced by high infant mortality, and the life expectancy of people who lived to adulthood was much higher. A 21-year-old man in medieval England, for example, could by one estimate expect to live to the age of 64.
Also, Vikings didn’t wear horned helmets, Romans didn’t puke in vomitoriums after rich meals, the average housefly lives for 20 to 30 days, medieval Europeans didn’t believe the Earth was flat, Napoleon was taller than average, the Bible’s forbidden fruit was not explicitly an apple, and humans have more than 20 senses. (via @linuz90)