In Korea (and in other Asian countries as well), it is a good idea to wear socks without holes. In many restaurant (and temples) you have to take off your shoes before entering.

Fortunately, I knew this before my trip to Korea:

http://es.news.yahoo.com/29012007/24/foto/in-this-combo-picture-world-bank-president-paul-wolfowitz-with.html

World Bank president Paul Wolfowitz, spotted while leaving a mosque in Turkey