Wabi-Sabi
In traditional Japanese aesthetics, wabi-sabi is a world view centered on the acceptance of transience and imperfection. Japanese do not really admire perfection ... The idea is that nothing in the universe ie perfect, therefor you should not look for perfection.
So when you design things, you leave something to make sure it is not perfect. The picture above is a gate at Nikko called Youmei Mon. It was so beautiful and prefect, the architecture decided to build the gate with one of the pillars upside down. That's wabi-sabi.