Always look for alternative explanations of the unbelievable.
This actually happened. A guy sent out 1000 letters to potential investors. In 500 he predicted stock X would go up. In 500 he predicted it would go down. When it went up, he sent letters to the people who received the accurate prediction. Again he picked a stock, and sent half a letter saying it would go up and half another saying it would go down. Then he repeated this process 4 more times. Eventually he wound up with a pool of 30+ people who he’d convinced that he’d “figured out the stock market”, because those 30 people had seen 6 accurate predictions in a row, and knew nothing of the other 970 people who’d received wrong predictions. Always look for alternative explanations for situations.