What's something you've seen once but you'll never see again?

I've been somewhat lucky in my career in the defence industry: I watched one of the last Concorde aircraft do a circuit around Bristol/Filton and come into land at what was then Filton Airport and land for the very last time - the main road was closed when it came in. There were a couple of thousand people at Rolls Royce who watched it and many of them were in tears as they had worked on the engines that powered it. Years later I worked for Airbus UK and that Concorde had been sat neglected for years where it had been parked. It was degrading on the apron, but, if you asked, you could mount the steps and go onboard. I've sat in the pilot's seat and gone back into the passenger cabin. It's now ben restored and you can visit it for a princely sum. I also worked for Westland helicopters in Yeovil back in the day. One time I was in a traffic jam on the A303 and a Harrier jump jet, for whatever reason came in to hover right alongside the road. I recognised the tail number as being one of the aircraft from the Boscombe Down test and development centre and waved at the pilot. The Harrier is famous for requiring "3 hands" to control it in the hover, but to my astonishment the pilot waved back! That was pretty cool. When I was at Westlands I'd take a shortcut through the machine shop, get a baguette from the bakery and wonder back outside past the airfield and a have 10 minutes to myself. I'd see a few helicopters going through their paces above the airfield, but on this one occasion I watched an Apache WAH-64 do a few wing-overs and then came to a hover about 100m from where I was walking. The Apache has a 30mm chain gun. It's a 2 pilot craft and the chain gun can be "slaved" to the gunner's helmet sight as well as the pilots. I was nonchalantly eating my baguette looking at the Apache, when I noticed the chain gun was tracking me. I raised my left hand and waved - the Apache bowed its nose in acknowledgement. The test pilot flying the helicopter was tracking me (eating my baguette) and when they saw me wave (which meant they knew that I knew that I was being observed) decided to acknowledge this by dipping the helicopter's nose. Cool, but very frightening, because that gun is awfully lethal