According to the US Federal Code of Regulations:
“The term natural flavor or natural flavoring means the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate, or any product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof, whose significant function in food is flavoring rather than nutritional.”
Now that is a pretty broad category of potential products that the food industry is able to classify as "natural flavoring." Let's just look at a few examples:
Castoreum: This is a secretion from the anal sacs of the North American beaver anus. Castoreum is a flavor enhancer added to improve the flavors of vanilla, raspberry, and strawberry. It is found in flavored teas, yogurt, ice cream, candy, and fruit drinks. The first time I found this referenced, I did not find it plausible so I went to snopes.com and yes, it was confirmed. I even found a website to order it in bulk, castoreum.com - freeze dried and smoked beaver rectum from AgroLaboratory, a food industry bulk supplier.
MSG: Products that contain MSG must list it on the label but there is a hidden way to pass it off. Companies only have to list the MSG but they have several ways to disguise it. Here is a list of some of the most common names for disguised MSG:
Hydrolyzed Vegetable Protein
Hydrolyzed Plant Protein
Plant Protein Extract
Textured Protein (Including TVP)
Hydrolyzed Oat Flour
Food Additives That FREQUENTLY Contain MSGMalt Extract
Natural Beef Or Chicken Flavoring
Food Additives That MAY Contain MSG Or Excitotoxins
Soy Protein Concentrate
Soy Protein Isolate
Whey Protein Concentrate
Benzaldehyde - Almond flavor: A frequently used process is to extract constituents out of peach pits but the process has been shown to create hydrogen cyanide which can be found in detectable levels. Interestingly, the same flavor can be created by a process that eliminates the cyanide but then it will have to be labeled as an artificial flavor.
It is important to be aware of what we eat but the simplest way is to eat foods that do not contain an ingredient list in the first place.