In 1720 Isaac Newton had the good fortune to invest early in the South Sea Bubble, making a quick and decent profit. Satisfied with his gains, he exited before the bubble fully inflated. However, as he saw his friends amass incredible wealth, he couldn't resist re-entering the market. In an attempt to make up for lost time, he invested far more—some of it borrowed—and, unfortunately, bought in just before the bubble burst. As the stock plummeted, he lost almost everything, with his investment returning to roughly the value of his initial, smaller stake. It's said that Newton, reflecting on his experience, remarked, "I can calculate the movement of heavenly bodies but not the madness of men."
Sir Isaac reportedly lost the equivalent of $4 to $5 million today, which amounted to almost the entirety of his investment in the South Sea Company. While this was a huge blow to his wealth, it did not leave him destitute, and he still maintained a fortune, though his stake in the company was essentially wiped out, losing around 90% of its value.