For all of those who "sat in the wrong seat by accident," we don't believe you. Sure it can and does happen. However, what a coincidence you sat in the aisle instead of the middle. Somehow, the reverse doesn't happen as often.

Luckily, I have not had to deal with this yet. If I did, I would nicely say "I believe this is my seat". If that didn't work, but I got a similar aisle seat, no sweat. If it meant a window, it depends. If trying to be forced into a middle, I would quickly get a FA involved if they didn't move.