I have only encountered an outright "poacher" once, in the F cabin of a domestic flight in the USA. When I gave her a baffled look, she gave me her rehearsed spiel of "We were (supposed to be) together, but 'it got messed up'," implying that somehow they involuntarily de-coupled after their booking, but before my seat assignment (quite a bit earlier). I took her seat rather than deal with evil vibes sitting next to her husband.