This has the potential to cause significant kashrut problems, both for Pesach and year-round. Examples of concerns include:
* Substitution of one kind of fish for another, so that one may not actually be eating kosher fish;
* Addition of grape juice to pomegranate juice, creating stam yeinam problems;
* Addition of corn syrup to honey, creating Pesach problems for those who avoid kitniyos.
Excerpt from the article:
The new records show that the most commonly fraudulent products are olive oil, milk, saffron, honey and coffee.
Tea, fish, clouding agents (used in fruit juices, like lemon, to make products look freshly squeezed), maple syrup and spices (turmeric, black pepper and chili pepper) were also top imposters.
Most of the reported food fraud was committed by producers adding fillers (i.e. other plant leaves to tea leaves), mixing in less expensive spices with high value spices or watering down liquids. Olive oils were often replaced and/or diluted with cheaper vegetable oils. Clouding agents were found in 877 food products from 315 different companies. Another popular target: Pomegranate juice, often made with grape skins and grape and pear juices.
The article also points the reader to a Food Fraud Database.
I'm not sure of what this means, but I found a record here, from 2012, of adulteration with "Milk from non-authentic animal species". I wonder what that means... And another report on the site identifies pork lard in milk powder! [Other reports have goat milk and water buffalo milk and "cow tallow" mixed in.] Requiring a hechsher on milk and milk powder is looking wise right now...
UPDATE: I stand corrected regarding the paragraph above; the findings mentioned in the previous paragraph include incidents from outside the US, as well as academic studies in which impurities were added for the sake of detection. I was duped by the fact that the site is run by the "US Pharmacopieal Convention" and the fact that I also include reports from the field.
Moral of the story: Even if an apparently simple product shouldn't require a hechsher (kosher certification by an outside supervisor), it may need it due to industry fraud.