Another interesting one is when we "knew" how the sun worked. Of course we didn't know about nuclear reactions until the early 20th century, so it makes sense we didn't figure it out for a while after that. But before that it was a complete mystery! Scientists had calculated how much energy would be required to produce the amount of light we see and it seemed impossible given the estimated mass of the sun, for any know physical method to produce it (chemical, gravity compression). Some thought it was made of coal (sign of the times I guess) but that would of course require oxygen to react with... And would have only lasted a few million years I think...