Every now and then we come across a deeply ingrained falsehood or half-truth in places that otherwise seem authoritative, and we wonder how that came to be. I think they spread in a manner very similar to "urban legends", where each authority takes a previous authority as its basis, without independently checking the conclusion. When a slight variation is included each time, this process can quickly lead to outright falsehoods. My favorite is the explanation often given for why high-mass main-sequence stars are so much more luminous than low-mass main-sequence stars. The typical story is that nuclear fusion is highly sensitive to temperature, and massive stars have higher temperature cores so that the pressure can support the greater mass of the star, all leading to the high luminosity. The logic of that causation is completely false, the sensitivity of nuclear fusion to temperature only tells you an estimate of the core temperature, the luminosity then follows from the least sensitive dependences on core temperature-- the overall force balance, which sets the stellar radius, which sets the luminosity, which refines the core temperature. The usual logic is exactly backwards-- things that are sensitive to temperature are not set by temperature, they set the temperature. I can only imagine that the many authors and websites that give the false argument are taking it from each other. What other examples of particularly unfortunate effects of this process can people cite?