Some stupid things that smart people do...

A few of my favorite stupid things smart people tend to do:

• Ignoring the importance of design and style - When the iPod originally came out, technical people complained about its lack of features and perceived high price ("ooh, who cares about another MP 3 player, I can go buy one at Best Buy for $50.  In the meantime, it was so cool and easy to use that normal people went out in droves to buy it.
• Using terrible tools, and taking pride in their awfulness - Especially common with programmers, who take pride in using programming languages and text editors that have been designed by programmers, not updated since the 1970s, and never touched by anyone with a modicum of design sense. They believe that mastering arcane, over complicated commands and processes are a mark of pride, rather than a waste of time.  I will refrain from singling out specific programming languages and tools here, because smart people also like to get caught up in pointless flame wars about this sort of thing.
• Following the pack - Many smart people often seem to be followers, probably because they grow up spending so much time pleasing others via academic and extracurricular achievement that they never figure out what they really like to work on or try anything unique.  Smart people from top schools tend to flock into the same few elite fields, as they try to keep on achieving what other people think they should achieve, rather than figuring out whatever it is they intrinsically want to do.
• Failing to develop social skills - Some smart people focus exclusively on their narrow area of interest and never realize that everything important in life is accomplished through other people.  They never try to improve their social skills, learn to network, or self promote, and often denigrate people who excel in these areas. If you are already a good engineer you are going to get 10x the return on time spent improving how you relate to other people compared to learning the next cool tool.
• Focusing on being right above all else - Many smart people act as if being right trumps all else, and go around bluntly letting people know when they are wrong, as if this will somehow endear others to them.  They also believe that they can change other people's minds through argument and facts, ignoring how emotional and irrational people actually are when it comes to making decisions or adopting beliefs.
• Letting success in one area lead to overconfidence in others - Smart people sometimes think that just because they are expert in their field, they are automatically qualified in areas about which they know nothing.
• Underrating effort and practice - For smart people, many things come easily without much effort.  They're constantly praised for "being smart" whenever they do anything well.  The danger is that they become so reliant on feeling smart and having people praise them, that they avoid doing anything that they're not immediately great at.  They start to believe that if you're not good at something from the beginning, you're destined to always be terrible at it, and the thing isn't worth doing.  These smart people fail to further develop their natural talents and eventually fall behind others who, while less initially talented, weren't as invested in "being smart" and instead spent more time practicing. 
• Engaging in zero sum competitions with other smart people - Many smart people tend to flock to fields which are already saturated with other smart people.  Only a limited number of people can become a top investment banker, law partner, humanities professor, or Jeopardy champion.  Yet smart people let themselves be funneled into these fields and relentlessly compete with each other for limited slots.  They all but ignore other areas where they could be successful, and that are less overrun by super-smart people. Instead of thinking outside the box, smart people often think well within a box, a very competitive box that has been set up by other people and institutions to further someone else's interests at the expense of the smart person.
• Excessively focusing on comparing their achievements with others- Smart people who have been raised in a typical achievement-focused family or school can get anxious about achievement to the point of ridiculousness.  This leads to people earnestly asking questions like: Success: If I haven't succeeded in my mid 20s, could I be successful in the rest of my life? and Are you a failure if you are not a billionaire by age 30? What about 40?
• Ignoring diminishing returns on information - Smart people are often voracious readers and can absorb huge quantities of information on any subject.  They get caught up in reading every last bit of information on subjects that interest them, like investing, life hacking, or tech specs of products they're planning on buying. While some information is useful in making a decision, poring through the vast amount of information available online can be a waste of time.  They end up spending a lot of time gathering information without taking action.
• Elitism - Smart people often use smartness as measure of the entire worth of a person.  They fail to see the value in or even relate with people who are different.