Showing too much skin. Except perhaps in Singapore, Bangkok, and other heavily touristed areas -- where modest Southeast Asians are more accepting of beachwear at the beach and sexier attire at discos -- dress with respect for the locals and their traditions. And don't wear shorts or short skirts to a temple or mosque -- it'll get you tossed out.
Photographing a member of a hill tribe without permission. There's nothing a visitor wants more than to take away indelible images of the colorful, rustic lifestyles of the Southeast Asian ethnic minorities. However, many rural people are superstitious about photographs or might resent the intrusion of privacy. Ask first.
Losing your temper in Laos, Thailand, or Vietnam. Many people follow Buddhist traditions in their daily life, approaching even unfortunate events with calm cheerfulness. They would be shocked and dismayed at anger or ill temper, and raising your voice won't achieve any purpose whatsoever. No matter how frustrated you become, keep it under wraps, or the people around you will see to it that you never get what you need.
Using offensive body language. Muslims, Hindus, and Buddhists all reserve the left hand for "unclean" toilet duties, never for pointing at anyone or anything, handing objects to others, eating, or touching other people. Similarly, in Buddhist and Hindu cultures, the head is revered as the most sacred part of the body, while the feet are the lowest. Never touch another person's head or shoulders, and never point or gesture with your feet.
Looking (or being) poor in Singapore. You probably won't run into too many cultural faux pas in cosmopolitan Singapore, but poverty is the pits in this city. Bring your smartest clothes if you want to impress people here.
Not belching after a meal in Bali. Releasing a good burp after a meal is considered a compliment to the chef.
