I accidentally sat in someone else's seat and he ended up taking mine. I wouldn't normally care to swap a window for a window, but it was an exit row. When I first sat down, I was initially surprised to find myself in a "regular" set because I thought I booked an exit row (and it turned out that I was right). I figured they just swapped planes like they did to me last week, but in fact, I mistakingly sat in the seat number from my previous leg. Anyway, long story short, I looked at my boarding pass, told him my seat number , and started to pack up to move. However, rather than waiting for me and without even asking if I minded, he sat down in my exit row seat, turned around and said, "Thanks!" He then giggled ... I was in shock and didn't know how to react. It was one of those situations where someone does or says something so blatantly rude that you're rendered speechless. In the end, I bit my lip, but what would you have done?