According to this AP report, the city of Aliso Viejo, California nearly banned styrofoam cups after learning that dihydrogen monoxide is used in their manufacture on the grounds that dihydrogen monoxide is a substance that could "threaten human health and safety". The gaffe was apparently the result of a paralegal being misled by a prank website that described dihydrogen monoxide as a tasteless, odorless chemical that can be fatal if accidentally inhaled.The description is perfectly true but misleading: dihydrogen monoxide is water.
At first I thought that this was just another depressing example of how little most people seem to learn, or retain, about science, but on reflection, I think it is more interesting. The fact that "dihydrogen monoxide" refers to water is not a fact that they should have learned in science classes because scientists don't normally refer to water that way. The problem lies in their ignorance of the system by which chemists refer to chemicals.
Every compound has a chemical formula and scientific name. The name is generated by a set of rules from the chemical formula. A compound may also have a common name. In the case of salt, for example, we have the chemical formula NaCl, the scientific name sodium chloride, and the common name salt.
I suspect that the problem is that most people know that scientists have exotic names for chemical compounds but that a lot don't understand the difference between chemical names and chemical formulae and that the names are systematically related to the formulae. The result is that, on hearing "dihydrogen monoxide" they don't even try to translate it into a chemical formula because they don't realize that there is a procedure for such translations, or if they do know, they don't remember how to do it. Furthermore, it doesn't occur to them that "dihydrogen monoxide" might be water because they think they already know the scientific name for water: H₂O, which they don't realize is a formula, not a name. So on encountering an unfamiliar term like "dihydrogen monoxide", they assume that it must be an unfamiliar chemical.