La Gabelle was the most hated tax in France - indeed, it was so hated that it was one of the reasons that King Louis was not just shuffled aside into a Constitutional Monarchy role, but had some Quality Time with Madame Guillotine. You could call the tax a mistake.Borepatch posts a history lesson regarding forcing a people to buy something by government decree. La Gabelle
So what was it about la Gabelle that made it so despised? After all, it was just a tax (in this case, a tax on salt). You don't like the tax, don't buy so much salt. Ah, there's the rub.
You had to buy the salt. By law, everyone ten years old or older had to buy salt, at a price set by the Crown. More infuriating, different parts of the realm paid different tax - favored regions got a break. Naturally, this led to a thriving smuggling trade, with increasingly draconian penalties as time went on. In the end, the cash was too important to the financially-strapped Ancien Regime to allow for reform, and the whole system collapsed.