In my fiery youth after putting up with an 11 hour delay at ORD, I boarded a LHR bound MD-11 minutes before closing to discover the coach cabin practical empty. Of course, according to Sod's law despite the empty cabin there was a gentleman already seated in the H seat next to my J seat. After 20 minutes, I excused my self and took an empty window pair two rows further back. Some minutes later, an elderly couple entered the cabin and walked towards my row, stopped and ever so politely notified me that I was sitting in their seat. With embarrasing petulance, I suggested they pick one of the other vacant rows. I know it wasn't clever and although the departure had turned into a total farce, I picked the wrong moment to have a strop in front of the flight attendant - certainly didn't help the situation. I was inches away from getting booted off the flight. Anyway, I guess that learned me and I promise you all, when travelling I now conduct myself in an appropriate gentlemanly manner.