I was trying to get
ChemicalData["C18H30O2", "MolarMass"]
which returns 9 values
{277.43g/mol,277.43g/mol,277.43g/mol,277.43g/mol,278.44g/mol,278.44g/mol,278.44g/mol,278.44g/mol,278.44g/mol}
of which only last 5 are right.
Looking why this could happen, it seems like Mathematica tries to get all known isomeres for "C18H30O2" but apparently several of them have invalid data.
That is, ChemicalData["C18H30O2"]
returns
{crepenynate,alpha-eleostearate,calendate,punicate,linolenic acid,1,2-dihexyloxybenzene,11-trans-13-trans-15-cis-octadecatrienoic acid,gamma-linolenate,4-dodecylresorcinol}
Which is right, but again Map[ChemicalData[#, "MolarMass"] &, ChemicalData["C18H30O2"]]
gives wrong molar masses
{277.43g/mol,277.43g/mol,277.43g/mol,277.43g/mol,278.44g/mol,278.44g/mol,278.44g/mol,278.44g/mol,278.44g/mol}