I was travelling in Australia on QF SYD-MEL a few years ago, it must have been the 25 Jul or so, and my BP said 8A... I boarded the plane, and found someone sitting in my seat! Politely I said "Excuse me, I believe you are sitting in my seat..." and showed him my BP.

Imagine my surprise when he pulls out his BP and it also said 8A on it, for the same flight! We looked at both of our BPs, and sure enough, they were identical, bar our names on them... So we summoned some CC and asked for some help. She also was befuddled and at a bit of a loss.

It took us several minutes to realise that I had pulled out a boarding pass for the same flight number and the same seat, but on 25 Jun, i.e. a month before! As it turns out, I had indeed taken that very same flight a month before, and had 8A assigned at the time... Obviously I had left the old BP in my passport holder and then pulled it out accidentally, instead of the new BP which was valid for the present day, which showed my correct seat number: 9A.

Oh well!