Another example: Miasma Theory
Scientists in the past used to used this theory to explain why people got sick. Scientists explained that the reason people fell ill were because of "bad air", that spread illnesses to people.
This was a good explanation, because it explained for why more densely populated areas got sick. Scientists said that it was because there was more bad air created from factories.
However, this explanation is untrue, because we now know that illnesses are living organisms, that we refer to as "germs", instead of arising from "bad air".