A friend of mine apparently made a major snafu while visiting a small village in India a couple of years ago. Some of the guys he was travelling with had family in the village and there was a celebration for their homecoming. At some point in the evening, he offered his left hand to one of the elderly men to shake (he had a glass or some such in his right). The man took it as a total affront and his buddies had to do some quick talking to keep him from being tossed out on his ear.

What he was told by his friends was that the left hand was considered unclean and by offering it, he had offended the old man.

I don't know how old the right-hand/left-hand custom is. Maybe someone else has something to say/add about it. I just know that my friend now uses this tale as part of his "party conversation" arsenal (so I've heard it several times). ;o)