Common misconceptions

A sampling from Wikipedia's fascinating list:

• Contrary to the popular image of the Pilgrim Fathers, the early settlers of the Plymouth Colony did not necessarily wear all black ... Black dye was expensive and faded quickly, and black clothing was reserved for the most formal occasions (including having one's portrait painted), for elders in a community, and for those of higher rank. 
• Cinco de Mayo is not Mexico's Independence Day, but the celebration of the Mexican Army's victory over the French in the Battle of Puebla on May 5, 1862. Mexico's Independence from Spain is celebrated on September 16.
• It is commonly claimed that the Great Wall of China is the only human-made object visible from the Moon. This is false. None of the Apollo astronauts reported seeing any specific human-made object from the Moon, and even Earth-orbiting astronauts can barely see it.
• "Edelweiss" is not the national anthem of Austria, but is in fact an original composition created for the musical The Sound of Music. The actual Austrian national anthem is "Land der Berge, Land am Strome."  
• The forbidden fruit mentioned in the Book of Genesis is commonly assumed to be an apple, and is widely depicted as such in Western art. However, the Bible does not identify what type of fruit it is. The original Hebrew texts mention only tree and fruit.
• Human blood in veins is not blue. In fact, blood is always red due to hemoglobin. Deoxygenated blood has a deep red color, and oxygenated blood has a light cherry-red color.  
• Swallowed chewing gum does not take seven years to digest. In fact, chewing gum is mostly indigestible, and passes through the digestive system at the same rate as other matter.

(h/t Kottke)