I do my own flight bookings and had some 12 odd sectors to fly and I forgot about one sector as I had not noted down and kept the bookings in my email and forgot about it and booked the same sector on a different airline but on the same day. I could not cancel it as it was a low cost booking in Asia so I took the higher fare flight. Now I have started using an app to keep a record of my bookings. I did end up at the wrong airport once to get my connecting flight. Actually I booked my connecting flight from a different airport and city to my arriving flight.
I did know of once incident to someone I know who actually was a diplomat who got agitated just before departure on his national carrier after they pushed back from the gate and had to come back to the gate as he forgot his passport. All the while he was sitting on it. He thought he left it behind at the Lounge.