Doesn't just happen on planes. A number of years ago, a colleague and I were on a train with our client down to a meeting. In the same carriage as us were a group having a loud conversation that turned out to be all about the subject of the meeting we were going to. They were advising the other side. By the time the train arrived at the station, we knew their strategy, their thoughts about their client (which were not complimentary), the defects in their arguments and the position they were willing to advise settling for.

Best meeting of my life.

1F