"When in Rome," I have learned the hard way, requires a keen eye for detail, and a bit of focus and awareness of everything you are doing when local etiquette is very different from your own.

The first time I ate as a house-guest in Japan(my third time being there), I passed a sample of food from my plate to one of my companions. This in itself is usually acceptable, and had been encouraged in this case as our hosts had prepared a broad sampling of "Japanese Home Cooking." However, which end of the chop sticks one uses to do this is important. They politely reminded me of this. I thanked them for the reminder. Not more than two minutes later, again distracted, this time by conversation with a man who spoke four and a half languages(the half being English) I stuck the sticks into my rice bowl. Imagine you neighbor's expression if, while at the table, you suddenly shot his dog.

I already knew this was an insult, but I had stopped being mindful of what I was doing and where I was. Fortunately, they were forgiving and understanding, but it didn't ease my shame in doing something insulting when I was already very aware of the custom.

Thankfully, most people are forgiving of mistakes made by foreigners. With the exception of traffic cops, but that's a different issue.