Before 1972, scientists believed that the rings of Saturn were composed of gasses or dust; on tests, any student who claimed that Saturn’s rings were composed of solids was incorrect because the true answer was that the rings were a gas. Later scientific investigation has shown that the rings are, in fact, composed of solid matter. Did the truth change? Should these students retroactively be given credit? Which group of students originally knew the correct answer?

Similarly, in the 1950s and 1960s physicians knew that stress caused ulcers, although we now know that bacteria causes ulcers. What does this imply for our definition of the words truth and knowing?

http://plato.stanford.edu/entries/truth-correspondence/