Exodus (2) - S1-E24
Corrected entry: At the very end of the episode, when the passengers are boarding the doomed flight in a flashback, there is a shot of Sawyer walking down one of the aisles. If you look closely, you can see his bag bumps someone on the shoulder, and they turn around to glare at him. It's Sayid, sitting on the aisle seat of the left section of seats. However, not too long later, we see a shot of Sayid sitting down in 'his' seat, but he is now sitting on the aisle seat in the MIDDLE section of seats.
Correction: It is not impossible to switch seats in an airplane. Maybe Sayid was unhappy with his seat and asked a stewardess if it was possible to switch, or maybe he gave up his seat to someone who asked him (if they were travelling with a companion and were seated apart form each other, for instance).
Twotall