Lee's answer[0] is a good one, and worth a look, but there are other good answers there as well. Andrea Martin's answer[1] just below Lee's is especially interesting because it takes an entirely different viewpoint to the question than the typical "intellectual biases" or "networking/superficial social skills" type of answers:
"Fail to develop and maintain mutual support systems among mature, benevolent, competent people. Without a good support system, anyone can begin to slide down a slippery slope when they encounter hardship, miscalculate something major, or fall victim to the misdeeds of others."
"Fail to develop and maintain mutual support systems among mature, benevolent, competent people. Without a good support system, anyone can begin to slide down a slippery slope when they encounter hardship, miscalculate something major, or fall victim to the misdeeds of others."
Go read the whole thing. It's worth it.
[0] Permalink to Lee's answer: http://www.quora.com/Human-Behavior/What-are-some-stupid-thi...
[1] Permalink to Andrea's answer: http://www.quora.com/Human-Behavior/What-are-some-stupid-thi...