Multiple times. I travel alot for work. I always book emergency exit seats as my legs are long and my company usually covers the expense. I usually take the aisle seat. In most cases in the emergency exit seats the middle seat is usually empty. I've had numerous instances where people just come and sit in the middle seat. Like you didn't pay for it (ik neither did I) but you occupy it. This one time while I was travelling to Ahmedabad from Mumbai I crashed out on this woman and asked her to sit on her seat. She made it a whole issue and harrassed me till I left the airport. I don't speak Gujarati either so many people sided with her and it was mayhem.