2) They can be really good at coming up with excuses for failure.
Just as you can use a gun for target practice or a robbery and a knife to cut a steak or slash a tire, intelligence is a tool that can be used many different ways. One of the most common ways brilliant people hurt themselves is by using their intellect to devise excuses for why they’ve failed instead of coming up with new ways to succeed. People who are really good at this can come up with a theory about life, see it fail every test, and still be just as convinced they were right as when they started. These are the sort of people Talleyrand once described as having “learned nothing and forgotten nothing.”