Mistaking Northolt for Heathrow
I sat in Northolt tower one blustery morning nearly half a century ago(!) when Heathrow were using 23L (now a taxiway) and was surprised how close and low their traffic was when crossing the final approach for 26 at Northolt. Later that same day, after I'd left, as passenger in an Aztec, Air France made their notorious mistake!
Just to be clear, the NO and LH arrows were added to the respective gas holders after that time, I think, 1964. The Uncle Roger column in Flight Magazine did a good parody about how NO meant "no" if you wanted Heathrow but NO meant "yes" if you wanted Northolt and LH being "yes" for Heathrow but "no" for Northolt!