On a Singapore Airlines flight a couple of years back, I saw a passenger in the departure hall and fervently hoped that he wasn't going to be between me and any exit I might need as he was absolutely enormous (horizontally, not vertically).
He was seated on the rear row of seats at the back end, spreading over two complete seats and into the third. Apart from the normal belt, he had two child belts attached together.
The people I felt sorry for on this 13 hour flight were those sat on the row in front of him, as they couldn't recline their seats at all - his gut was pressing against them! He didn't leave his seats for the whole flight, and trying to access a toilet would have been a joke.
I was hoping that he would have had to pay for the extra seating, and/or the passengers in front got a refund - I didn't find out, but was grateful that I was sitting further away.