The answer is:
They invest more money than they can afford.
They focus heavily on familiar investment opportunities.
They hold onto investments longer than they should to recoup losses.
They put all of their money into one kind of investment at a time
Investing more money that you can afford could directly bankrupt you if the investment is somehow failing.
Familiar investment opportunities tend to attract a lot of people. This could cause the value of your investment to fall because many people are buying it.
Often times, smart investors need to aware when they should acknowledge loss and get out before too late.
Smart investors would diversify their portfolio. If one of their investment fail, they can still have a chance to recoup the loss by allocating the profit from other investment.