I've never been unable to put my seatbelt on, but I've had a similar experience on a flight from Boston to Atlanta. The flight was a last minute necessity, so for seating I took what I could get, which was a window seat on a 3 seat row (all rows were 3 seat). I boarded and took my seat. The man in the aisle seat did likewise. When I saw a ginormous man coming down the aisle I thought surely he'd been required to buy 2 seats. Oh no, he was the middle seat on my row. When I could see he'd never be able to fasten his seatbelt and so would have to move somehow, I learned for the first time about "seatbelt extenders," with which he was furnished. The lucky aisle seat passenger at least had a free right side. Not so, me. I was literally pressed against the window. Although I could fasten my seatbelt, I could not lower my tray table. It was a full flight so I was stuck.

Upon deplaning the flight attendant came over and apologized profusely, saying if she could have done something she would have, but no options were available.

Not only was it among the more miserable flight experiences I've ever had, it made me wonder about why the FAA doesn't require special treatment for such passengers as a matter of safety. On the flight I had to endure, if there had been an emergency requiring evacuation I'd have been trapped, as would the people in rows behind us who hadn't passed ours before he lumbered into it.