FlyerTalk Evangelist
Join Date: Feb 2009
Location: From ORK, live LCY
Programs: BA Silver, EI Silver, HH Gold, BW Gold, ABP, Seigneur des Horaires des Mucci
Posts: 14,214
If it happens to me, I recheck my boarding pass to ensure I haven't screwed up (which I have a few times) and then politely say something like "excuse me, I think that's my seat". In the unlikely event that it is contested by the seat occupant, I escalate to the nearest cabin crew member. Haven't got beyond that yet.