Category: things of which things are not made.
http://en.wikipedia.org/wiki/Category:Misidentified_chemical_elementsThe best one is Gnomium.
(Scene: a chemical laboratory. Day.)S
CHMIDT: Herr Professor Krüss, we have a problem. Cobalt has these chemical properties, and this weight, and you will at once see that by our theory it is too heavy to have this chemistry. What can we about this do?
K
RÜSS: That is quite all right, Herr Professor Schmidt. If cobalt has these properties, then by our theory it must be lighter. So, then, our sample of cobalt must be mixed with something heavier but chemically indistinguishable. Let us call it Gnomium. Is sehr funny German joke, ja?
S
CHMIDT: Ach, I see. So, this Gnomium, it must have the same chemistry as cobalt, and is heavier than cobalt, and it is so because cobalt is already too heavy to have this chemistry. Do you not perhaps a teensy tiny contradiction in this see?
K
RÜSS: Hush! Let us publish quick and hope nobody notices.
(Exeunt to nearest Bierkeller.)