Over at io9, you can read about an element that was discovered in 1864, until it wasn't in 1927.
The element, called "nebulium," was first observed when scientists began using spectroscopy to detect elements via the light wavelengths they emitted when heated -- the light shining from them would be aimed at a prism, which would split it into its component parts. An unknown element could be heated, the light viewed through a prism, and then its identity discovered when the resulting readings were matched against known elements.
The same technique was used to figure out which elements were in stars. Their emitted light, viewed through a prism, created a spectroscope reading that showed what they were made of. This is the way helium was first discovered, for example. It was actually detected in space before it was ever known on earth.
Nebulium, though, presented a problem. Scientists had also been organizing elements based on certain characteristics of their atoms. These characteristics tended to group them together quite nicely, becoming the modern periodic table. But nebulium, supposedly created in interstellar gas masses called nebulae, didn't fit on the periodic table. So either spectroscopy was wrong, or the periodic table was wrong. Scientists didn't know what to do.
But in 1927, astronomers uncovered the secret: Nebulium was actually oxygen, only in a form unknown on earth. The oxygen in the nebulae had been "double ionized," which meant it was shy two electrons and it was highly energized trying to grab those electrons from anything nearby. When it lost that energy, it emitted a photon, which is what made the light.
Spectroscopy and the periodic table both turned out to be right, and nebulium went from being called "forbidden" because its spectroscopic signature was in a place elements supposedly couldn't be to being called "falsified" because it never really existed.
There's probably a lesson in there but I'm not sure what it is.