Impossible to get on wrong aircraft ? well, well....
We were sitting on the FR 737-800 parked at Girona and due to leave for Rome, some two years ago, when - after a head count - one of their charming cabin staff said "ladies and gentlemen, if there are any passengers who wish to fly to Paris, would they please disembark and get on the aircraft next to us, as THIS one is going to Rome..."
To applause (from the Catalans) and a brief chorus of "Why were they born so beautiful" (from the British - all right, from me), two red-faced passengers struggled down the aisle & steps and got onto the flight next door.
And I've done it myself, bording with no questions asked an AF flight from Charles de Gaulle to Dusseldorf, aircraft parked on the left, when I wanted to go to Frankfurt a.M., aircraft parked on the right. Only by chatting to the chap in the next seat did I find out. Caused no end of confusion when I got off, having to retrieve my boarding card from the AF stew who hadn't bothered to read it properly.
So not a bad idea, checking boarding cards on board, when they are actually looked at.
RaF