What is the most common flaw you find in humans?

Hypocrisy!

Or the fact that humans constantly lie, and some do it so well.

I dare all of you to tell nothing but the truth for a whole day.

They are ignorant and most lack practical rationality.

-Edit-

Satyendra, you've been misinformed about humans only using 10% of their brains, when in fact, a person probably uses 95-98% of their brains. They just don't uses some parts of their brains at certain times; it's not like you are trying to solve a problem, or solving a math problems at all times of the day.

Incredible arrogance--pretending they know things they barely have acquaintance of, pretending to abilities they almost completely lack, disrespect for scientists, real talents, etc., willingness to pronounce value judgment loudly when they haven't a clue what normative level of good is--such as about TV shows, actors, political aspirants, experiences, arts, scientific hypotheses, beauty.

This explains why I like dogs and trees and stars so well...

Greed

Flaws are what make us human. If we each didn't have imperfections, there would be nothing to highlight our fine points, right?

Selfishness

They exhibit wasted emotions. No one should ever hate,anger or feel jealous. I don't . My Mother taught me the secret of self discipline. Politeness and kindness should always prevail.

Judging other ppl flaws

For all the intelligence and technology they have, they still can't figure out how to not run over my cousins as they are attempting to cross the road.

Resentment.