A few months ago a work colleague and I were flying down to WLG. Another passenger got on and complained that my colleague was sat in their seat. However both had a boarding pass for the same seat but the person soon realised that they were sat in the seat indicated by their boarding pass for the same flight the previous week. They then moved to the correct seat.

When they were scanning their boarding pass at the gate they had got a red indication but the FA looked at the pass, had seen it was for the correct flight and allowed them to board without checking the date. Similarly at the aircraft door the check had only been for the flight number and not the date.