There are a few threads around on the common mistakes we make; some of the threads to do with cognitive biases etc. All good stuff.

This thread is motivated by a new book (that I haven't read yet) from an investment practitioner, which makes a nice change from so many of these articles and books etc. being from non-practitioners. Not that practitioners have a monopoly on wisdom and insight, its just a nice change.

The book is Think Twice: Harnessing the Power of Counterintuition, by Michael Mauboussin.

Here are links to two articles, including video interviews with Mauboussin.

8 Common Mistakes Investors Make
In summary (woefully inadequate), here are the 8 mistakes:
Succumbing to tunnel vision.
Being overly reliant on experts.
Not realizing how much we're influenced by peer pressure.
Viewing each problem as unique.
Illusion of understanding.
Assuming there's a definitive answer.
Assuming all risks are factored in.
Confusing luck versus skill.

Here is an expansion on the 'peer pressure' mistake:
Peer Pressure: Why Even Sophisticated Investors Succumb to Groupthink