Literally, most flavorings in e-liquid do not cause harm, but there are a select few that, while safe to consume, are not as safe to inhale. Responsible companies are going to do everything they can to provide vapers with quality products as well as producing quality e-liquids.
However, even the e-liquid companies cannot make studies materialize that do not exist. As scientists, as vapers just as general humans we have no idea what the potential harm is to inhale many of these so called approved flavorings, let alone what inhalation of the items will be like after vaporized.
It is deniable that flavorings are the essential selling speck of e-cigarettes. Besides that, e-liquid that consists of flavour chemicals declared as safe for ingestion by Flavor Extracts Manufacturers Association may not be considered as safe for vapers for use in electronic cigarettes. In addition, high exposure levels to some chemicals that are found in some flavouring could cause respiratory irritation to vapers.