Had exactly that happen on a flight back from BRU on Saturday evening. I was in 8F, could see from expertflyer that 8E was blocked and 8D was taken. Boarding was complete, doors closed and someone decided they wanted to sit in 8E. Not sure if it was off his own back or he asked the crew to be honest, but either way his boarding pass had 22A on it.

What made matters worse was he then decided to sit diagonally, so the guy in the aisle had to make do with his feet, where as I had the pleasure of his elbow.

Least it was only a 40 minute flight.