Things that "everyone knows" but which turn out to be false are a favorite bugaboo of mine. The latest XKCD comic paints a picture of a world where these things are specifically addressed by schools. I love this idea. The comic mentions a Wikipedia article listing common misconceptions, which I found to be a great read.
Note that I'm not claiming to be less susceptible to these things than anyone else. There are lots of urban legends that I've believed in at one time or another, and when someone sends me a terrified e-mail mentioning the latest one, I try to remind myself that we all fall prey to them now and then -- especially when they push one of our emotional buttons.