Why do smart people make big investment mistakes?

"Success in investing doesn't correlate with I.Q. once you're above the level of 25. Once you have ordinary intelligence, what you need is the temperament to control the urges that get other people into trouble in investing." Warren Buffett

Investor mistakes can greatly reduce your returns. Below are some of the most common ones. Add additional ones through your comments.

• Selling a security to protect your gain only to see it make a huge advance afterward.
• Holding on to losing positions waiting for them to return to your purchase price so you don't have to realize a loss and admit that you made a mistake.
• Resist buying a hot stock, market sector (like oil), or mutual fund, until it reaches heights you never thought possible and then you buy in—just in time for an immediate reversal down.
• Losing money in the market and swear you'll never go back in.
• Falling in love and buying at any price.
• Hating a stock and not buying in at any price.
• Selling early and watching the price continue to go up and not buying in again.
• Feeling the stocks you know (local companies or large companies whose products you buy) are less risky?
• Overly confident in your ability to outperform the market.
• No calculating your returns or comparing them to appropriate benchmarks?
• Not taking into account the cost of trading or taxes.
• Delaying participating in your 401(k) or other retirement plan?
• Acting on stock tips.
• Believing that media and industry commentators give you information with your well being in mind.
• Staying out of the stock market becuase it is too risky.
• Keeping too much money in savings banks and CD's.

Please add your investment mistakes through the comments dialog below.