I was once being shown around a museum in North Korea when another member of my party- a Finn who claimed to be a priest in the Church of the Flying Spaghetti Monsters (look it up) took off his shoe and started banging a large gong that was by the door. We barely escaped with our lives and were chased by an angry and murderous mob for a good mile out of town.
The Finn had broken not one, but three golden rules when travelling in Korea. Firstly, the sole of your shoe is seen as deeply offensive (as in the Middle East- remember the guy who threw his shoe at George Bush?). Secondly, the gong was an ancient treasure and absolutely not to be touched by dastardly foreigners. Thirdly we were in North Korea and you don't do anything without permission in North Korea.
This is an extreme example but you do need to be aware of local customs when travelling abroad or you could get into some serious trouble. Bedouin tribesmen are very hospitable and do not take no for an answer. My family once politely declined the offer of some tea in a Bedouin camp outside Palmyra in Syria as it was getting late and we needed to get back to the oasis before sundown. We were chased through the desert by a group of Bedouin in a truck who overtook us, stopped our car and forced us back to their tents for tea at gunpoint. You should also be careful when in the Middle East if you are left-handed. The right hand is used to eat with while the left is used... for toiletary functions. Do not attempt to shake hands with anyone with your left hand, as this is not appreciated.
More randomly, when in Russia and buying someone a bouquet of flowers, make very sure that there are an even number. It's considered very bad luck otherwise. In Asia, make sure that you hand anything to someone else using both hands- a casual one-hand pass is seen as immensely disrespectful as is tipping in Japan.
There are more obvious no-nos - Both New Zealand and Canada are immensely chippy about their more powerful and better-known neighbours - this is why they always have a flag on their luggage. Never mistake a Kiwi for an Aussie and never insinuate that Canadians are basically polite Americans. They will never forgive you.
The trickiest country of all is a surprising one that many Brits visit a lot. Thailand - the land of smiles - had an innumerable amount of things that you need to watch out for. They are incredibly protective of their king and so be very careful that you don't fold a photograph of him or step on his image (trickier than you might think since he is on the money). However exotic and kindly a Buddhist monk might look, do not attempt to put your arm around one and take a selfie, especially if you are a woman - you will regret it.
Similarly, do not make fun of the Buddha by attempting to take a photo with you looking like the statue. Locals will not be amused. The clothing restrictions in Buddhist temples can run to two or three pages- no shorts, no torn jeans, no see-through shirts, no shoes, no pointing at Buddha, kneel when taking a photograph... turns out that Buddhists aren't quite as relaxed as you might imagine...
Basically - just use your brain, do a little research and don't, whatever you do, don't strip off on the top of a live volcano... It's a known scientific fact that this sets off earthquakes.
Dom Joly's new book "Here Comes The Clown - A Stumble Through Showbusiness" is published by Simon & Schuster and is out now