Describe the “accident” that befalls the Monieur de Marquis in the streets of Paris.

The Marquis St. Evremonde recklessly orders his horse-drawn carriage to be raced through the streets of Paris at break-neck speed. He knows full well that someone could easily be killed but he doesn't care. The wicked Marquis positively hates the common people; to him, they're little better than animals. So if one of them should get run over by his carriage, then that's just too bad.
Inevitably, someone does indeed get run over, a young boy. The Marquis's response to this tragedy is to throw some coins out of the window of his carriage to the dead boy's father, Gaspard, as well as to Monsieur Defarge, the wine shop owner, who helps him. This arrogant gesture underlines the contempt that the Marquis, in common with most French aristocrats, has for ordinary people. Indeed, the Marquis is so contemptuous of the lower classes that he thinks that a grieving father can be bought off with a couple of gold coins. This whole tawdry episode illustrates the fact that in pre-Revolutionary France, the nobility can get away with the most serious of crimes.
As the Marquis drives off, the coins that he threw out of the window are flung right back at him. The Marquis is furious at this, and tells the assembled throng that he'd willingly drive over any of them. This heated exchange between a heartless, entitled aristocrat and the common people of Paris foreshadows what will happen during the French Revolution, when the poor, oppressed masses will rise up against the aristocrats who have exploited them for so long.