I actually did have the same thing happen in a 1st class seat several years ago. I'm thinking I'm safe because of the seat size... wrong! My seat mate was a very nice woman but she was probably 3.5-4X my size, thus was part way in my seat as well as in the aisle. I felt for her as she obviously had some medical issues. I appreciated that she had the common sense & courtesy to buy a seat that would better fit her size & was thankful the entire trip that we were not in coach. I still had some room left in my seat for me.