This is a matter of some debate, although there is general agreement among historians that Marie Antoinette never told her minister "let them eat cake.
Marie Antoinette had been born Austrian, a fact which had not endeared her to the French people. Additionally, she and her husband, Louis XVI, appeared to live an opulent lifestyle which was resented by the poor, who were on the verge of starvation. The desperate plight of the common people led to rumors of haughty comments by the nobility. A common rumor was that one noblemen had said the people should "eat grass, like my horses."
On October 5, 1789, thousands of angry peasant women marched to Versailles, armed only with farm implements. They murdered a number of palace guards and ransacked the royal apartments, presumably commenting as they searched for Marie:
We are going to cut off her head, tear out her heart, fry her liver, and that won’t be the end of it.
Even this did not bring about her final demise; the people had only insisted that Louis and Marie return to Paris, which they did. However, rumor spread that Austria planned to invade and restore Louis to the throne. Since Marie was Austrian, she was quickly blamed for the invasion. Louis was the first to be executed, and Marie soon followed during the resulting "reign of terror." It would appear that both were simply victims of the reign of terror, and because of their position, were the most prominent.