Saw an interesting article from New Scientist.
Growing up, you always hear about the Oort cloud. How comets come from there every once in a while, and kill dinosaurs and stuff.
Only trouble is, no one has ever observed the Oort cloud. It's just a made up idea to explain why the solar system hasn't run out of comets (since they burn up over time, and crash into stuff all the time).
Good to keep reminding us that there is no evidence for it, though.