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."

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...

Postfix those URLs with ?share=1 so us without Quora accounts see something useful.

[0] http://www.quora.com/Human-Behavior/What-are-some-stupid-thi...

[1] http://www.quora.com/Human-Behavior/What-are-some-stupid-thi...

Thanks for that. I can't edit my post anymore, so I can't fix it above.

Quora's really aggressive about getting people to sign up. I had to log in with a dummy account just to get the permalinks in the first place.

I'll have to take a look at that as well.