an example of overconfidence would be: Joe makes a stock price prediction and believes that there is only a 5% chance that his estimate is wrong; overlooking recent articles about the bad financial health of the business.
Buying stocks without any prior knowledge in finance would provide people with 50% rate of success. Good stock traders usually could improve their success rate up to 70% while success rate of 95% is very unlikely. Especially if the financial information showed a bad sign. The fact that Bill still put his money on the company's stock indicates that he is overconfident.