I once briefed an old guy (that I was sure was on the verge of senility) for about half an hour on the basic principles of flight only to find out ten minutes into the lesson that he had about 2000 hrs on Lancasters.
One other chap that wheeled himself through the door of the flying club in a Chair turned out to be an Emirates 777 Captain that had been disabled in a Car accident.
All present Instructors practice the phrase: "Hi. My name is .... Have you ever flown an Aircraft before?"