Was travelling on a BA flight from LHR - NBO in WT+ a few yrs ago, boarded the plane and found a woman sitting in my seat. The cabin was 3/4's empty, so decided not to make a fuss and moved 2 or 3 seats back. Doors closed, plane was pushed back and then 1 of the CC came over and asked to see my Boarding Card, I showed it to him and explained why I wasn't in my assigned seat.
Turns out the woman was traveling in WT and had just taken the first seat that she passed, regardless of cabin! The CC left her for take-off and then firmly escorted her to her correct seat.