Observational bias is interesting. For example, if you judge by the stars visible to the naked eye, you get a very different sense of the distribution of stellar masses than if you use a telescope, because no class M main sequence stars will be included (the brightest has apparent magnitude 6.7). The intrinsically brightest stars, including many of the famously named ones, are visible to great distances and so are very overrepresented.<p>The most spectacular form of observational bias could be the presence of life. The planet we are on will always have life (because, we are here and we are life) regardless of how uncommon life actually is in the universe.