The recent class-action lawsuit filed against Taco Bell, claiming that its "seasoned ground beef" doesn't contain enough beef (in addition to all the soy fillers, etc.) to actually be labeled beef, reminded me of the similarly shady history of tea's actual ingredients. Many tea histories I've read contain frightening but usually colorful accounts of the various ingenious, fiendish ways businessfolk have either stretched a meager supply of tea into a greater amount with additives or simply faked the product altogether.
Sometimes, used tea leaves were simply resold as new. Servants around Britain could earn a few extra pence by selling the contents of their masters' cold tea pots to firms that would dry them on hotplates and repackage them for sale. The 10th edition of the Encyclopedia Britannica, from 1902, reports that "according to Mr Phillips, of the Inland Revenue Office, there were in London alone, in 1843, as many as eight manufactories in which the exhausted leaves, obtained from hotels, coffee-houses, and elsewhere, were redried, and [adulterated] with rose-pink and blacklead, in imitation of genuine tea."
If no actual tea was around, some unscrupulous profiteers simply chopped up the leaves of whatever was available and labeled it tea. Roy Moxham, in Tea: Addiction, Exploitation and Empire, lists the impostors:
The favourite leaves used for adulteration were hawthorn for green teas and sloe for black teas; but birch, ash and elder were also used. Of course, the leaves of these trees did not make a convincing liquor, so it was necessary to add various colouring agents. In addition to the terra japonica [tannin from the acacia tree], additives included verdigris, ferrous sulphate, Prussian blue, Dutch pink, copper carbonate, even sheep's dung. Of these, sheep's dung was probably the least harmful.
An 1820 document, Treatise on Adulterations of Food and Culinary Poisons by German chemist Frederick Accum, contains a record of dozens of prosecutions and convictions for these crimes, based on a law passed by Parliament in 1725 specifically against the adulteration of tea. (The U.S. Congress got around to this in 1883.) One such case against a Mr. Palmer, who was busted with "a quantity of sloe leaves and white-thorn leaves, fabricated into an imitation of tea," discusses how the man and his associates would gather leaves from trees and bushes around town — "at the moment [victims] were supposing they were drinking a pleasant and nutritious beverage, they were, in fact, in all probability, drinking the produce of the hedges round the metropolis" — and then treat them with an "article used in producing the appearance of the fine green bloom observable on the China tea [but] was, however, decidedly a dead poison!"
The British, incidentally, started out drinking mostly green tea. They wound up as lovers of black tea (which took milk and sugar better) — and Moxham suggests the specter of lethal additives was the cause for the change in tastes: "The publicity given to the adulteration of tea, and the understandable public concern about the use of poisonous copper dyes in green teas, in particular, seems to have brought about the shift in consumption from green to black teas.
These are historical examples. I still see headlines in online searches sometimes about present-day adulteration incidents, mostly from India, and in the last couple of years there was an obvious effort to educate Indians about detecting adulterated tea and other foods (here's a lil' animated movie about it).