ATC watcher
Perhaps it was Flex but that was only the first of many flights in A340-600s that I made over a 4 or 5 week period. During that time I was also regularly flying in A330s. (Although I say it was from a pax perspective I was actually working on-board as a "flying spanner" on the IFE system.) I have flown on A310, A330, A340, B747, B767, B777 and all the usual narrow body types and the A340 is still the only one where I have had that disconcerting feeling.