3) They sometimes become overconfident about their intellect.
If dumb people have a tendency to ask too many questions and move too slowly, their more clever brethren can make the mistake of asking too few questions and plunging in too quickly. This can often backfire because brain power is not applied equally across all facets of a human mind. You can be brilliant at math, but average at English; have a knack for dealing with people, but be unable to understand computers; be a marketing wizard, but a relationship disaster.
Many smart people make the incorrect assumption that because they’re smart in one area, they’ll be just as smart in every area once they “figure it out.” Napoleon was sure he would “figure out” how to deal with the Russian winter; Bernie Madoff thought he would “figure out” a way to get away with fraud; and William James Sidis, who may have been the most intelligent man who ever lived, was probably shocked to end up in a sanatorium for a year because of his decision to participate in a socialist riot. Unfortunately, there is no such thing as too smart to fail.