Here are some Cognitive biases that I feel users who follow the message boards suffer from occasionally. Myself included.
1. Confirmation Bias
This is a fatal flaw of trading; we tend to surround ourselves with information that validates our own point of view and dismiss input that conflicts with our reasoning (also known as cognitive dissonance).
2. In-Group Bias
This is a manifestation of confirmation bias, or the tendency to surround ourselves with those who share similar takes on the stock. Not only does this provide a false sense of security in our individual viewpoints, it makes us suspicious—or angry—with outsiders who dare to question how we feel about our stock.
3. Post-Purchase Rationalization
One of our Ten Trading Commandments is that the definition of an investment should never be a trade gone awry. Nobody initiates market exposure expecting to lose money, but we should never post-rationalize our risk (such as ignoring stop-losses or throwing good money after bad). We would be wise to remember that good traders know how to make money but great traders know how to take a loss.