Maybe the biggest problem really intelligent people have is that they spend more time being clever than being effective.
I used to suffer from this disease of the mind. I'd want to do something new, novel, and fascinating - instead of just getting something done.
The really effective people I know, the people who make the biggest difference in the world, who make the best things, who get the most done, who live the best lives - they all are more concerned with getting something done that fits than with making it clever.
Over-researching relatively minor things is a great example. Take a quick look, get an understanding, choose one. Change later if it becomes an issue.
Trying to reinvent the wheel constantly.
Using words people don't know - maybe makes you seem clever, but makes you much less clear and less effective.
Trying to show off instead of listening.
Trying to make a point in an obscure way for no good reason - instead of just saying it outright.
The absolute worst one? Arguing with people over stuff that doesn't matter - you win the point, but become radioactive to whoever you were arguing with. Proving someone wrong? Yeah, clever. Very ineffective, though.