From http://news.yahoo.com/video/whoknew-historical-inaccuracies-060000279.html

If you know a little boy who wants a ninja costume for his next birthday, he might be shocked to find out that the outfit should be blue, not black. That’s because real nighttime ninja uniforms were typically navy blue, as that color blends into the darkness better than black does. And ninjas often dressed in normal wardrobe so they could blend into the crowd while spying. The image we have of ninjas as sword-swinging assassins comes more from movies and comic books than actual history. But ninjas aren’t the only case where our supposed “facts” about history aren’t true. Another warrior group, the Vikings are usually shown with giant horns coming out of their helmets. But historical evidence shows that the Vikings wore simple iron or leather caps without horns. The depiction of horns was likely added later by artists wanting to make the Vikings seem more fierce or barbaric.