Bill Miller, Legg Mason Value Trust's manager, used to be a good investor but he's outlived his usefulness in that role. Legg Mason (NYSE: LM) has kept him on for too long and if it doesn't give him the hook fast, he will sink the company. The problem? Miller's success has gone to his head and he can't adapt.

This phenomenon is quite common. It's called confirmation bias -- the tendency of decision-makers to seek out information that reinforces their views of the world and to reject information that challenges those views. This is particularly common among those who have been successful. They think that they have figured out a winning formula and when it stops working, they blame everyone but themselves.

This came to mind as I read a CNNMoney story on how Miller's fund has lost 34% of its value since last July. Miller had been famous for beating the S&P 500 every year between 1990 and 2005. But his methods have failed him since. And investors have yanked $2.4 billion from Legg Mason which CNNMoney notes, reported a second quarter loss last week.

If you are fortunate enough to have the money to invest in stocks, you may have made some money doing so. But you may also have made your share of money-losing investment mistakes. I know I have made plenty of such mistakes. Based on my experience, here are three that I would guess are pretty common:

• Not reading the prospectus. Too many investors buy stocks on tips from a broker or a TV stock promoter. They do not read the financial statements of a company. If they did, they would know about financial challenges, legal problems, industry uncertainties and other problems which could hammer their investments. But people don't read these financial statements, in many cases because they lack the financial education to make sense of the information.
• Not setting stop losses. People fall in love with a stock once they've invested. If the stock goes down, they hold on because they don't want to admit that they were wrong. Investors should set stop losses – if the stock falls 2% to 5% from the original price, they should sell. Most investors do not have the discipline to do this. But if they did, they would limit their portfolio risk tremendously. Would they also miss out on some opportunities? Probably, but more often than not, they'd save themselves losses.

The White House is in overdrive promoting former press secretary Scott McClellan's What Happened. That promotion has helped drive it to the number one selling position on Amazon.com (NASDAQ: AMZN). While Amazon will benefit from the sales of the book, the stock is more than fairly valued.

The White House's promotion is based on its passionate response to the confirmation bias it demonstrated in the run up to the Iraq war. As I wrote in this Business Strategy Review article, confirmation bias is when facing a major decision, one exhibits an unwillingness to admit conflicting data – no matter how salient – to influence a closed point of view. Mclellan points out that the White House decided to go to war against Iraq a year before its start and manufactured a false "case" to sell it.

McClellan pointing this out is hardly news. But I thought his comments about George W. Bush's cocaine use, as reported by 6abc.com, were more revealing. Recalling a 1999 conversation with Bush, McClellan writes: "'The media won't let go of these ridiculous cocaine rumors,' I heard Bush say. 'You know, the truth is I honestly don't remember whether I tried it or not. We had some pretty wild parties back in the day, and I just don't remember.'

Investors make mistakes every day. If they didn't we'd all be as rich as Warren Buffett and we're not.

Here's a list of six such mistakes:

• Follow hot tips. As a blogger on AOL's BloggingStocks, I know that some of the most popular posts are the ones that repeat what Jim Cramer said on his TV show five minutes before the post appears on the blog. The reason these posts are so popular is because lots of people are Cramer Ditto Heads (CDHs). He tells them what to do and they do it. While some use Cramer as a starting point for further research, many are too willing to be led and are not inclined to do their own research.
• Don't know how to research fundamentals. One of the reasons people don't do their own research is because they don't know how. Specifically, one kind of research many people don't know how to do is understanding how a company -- whose stock someone wants to buy -- fits within its industry. Many people would not know how to begin answering fundamental questions such as: Is the industry profitable? Why? How is that profitability likely to evolve? What is the company's market share? If it's a leader, can it sustain that leadership? If it's behind can it catch up? What kind of cash flow does the business generate? How much cash flow is it likely to sustain in the future? Does the market recognize these future cash flows in its price?
• Don't know how to analyze technicals. Many times fundamentals have nothing to do with how a stock performs. For example, in December 2003, Martha Stewart Omnimedia Inc. (NYSE: MSO) stock started going up from $9 when its Home & Garden Television (HGTV) show was taken off the air to $36 in February 2005 when Martha Stewart got out of jail. During that time the company saw its revenues shrink 20% a year and its losses skyrocket. The reason the stock went up is a mystery. But I thought people who were loyal Martha Stewart Ditto Heads (MSDHs) bought MSO as a show of support. Many investors do not know how to analyze money flows that would provide clues to what is driving a stock up or down. This can cause them to buy when they should be selling, or sell when they should be buying.