In 1975, out on one of those undulating plains they have in the UK, heard a bang from one direction and saw an airplane standing on its left wing through a gap in the hills. Then about 0.5 seconds later the same aircraft, same attitude, but 90 degrees away in azimuth. After that, nothing at all.
In 1997, had a window seat in an aircraft over an beautiful Indonesian island. Suddenly right below us there is a different aircraft, maybe 5000 feet down, and going the opposite direction. Spoke to a few people since and had the suggestion made that navigation with precision well below a metre is actually making some sorts of conflict more likely, because everybody follows the same line.