Many years ago I was flying as a student in an RAF Whirlwind helicopter. Towards the end of the sortie we were unexpectedly asked by ATC to go to search a particular area for an fixed wing aircraft whose pilot had called a Mayday (didn't find it, turned out they were actually many miles away in Snowdonia and had already crashed).
The SAR Wessex was on its way from RAF Valley to take over from us; we were almost out of fuel. To assist the crew to find us quickly, so they could resume from where we left off, I switched on our searchlight. Immediately I did so, a Jaguar appeared right in front of us, head on but entering a hard right turn, same level. Not only did we see it but we immediately heard it over our own aircraft noise and we smelt his exhaust fumes as there was no time to avoid his jet wake; we were bounced around by it as it passed just off to our left. It was a very close shave and it was investigated by the RAF. The Jaguar was not allowed to fly in our helicopter LFA; he shouldn't actually have been there. The pilot said he didn't see us, only the searchlight. He immediately pulled hard to avoid us and thought he had probably hit us anyway. We were both hidden from each other by a thin layer of haze on the inversion.
Another time I was invited to fly some aerobatics with a new aquaintance in a Steen Skybolt. I sat in the front seat. Our flight consisted almost entirely of aeros and went without incident. The next time that aircraft flew it never came back. It crashed and burned in the same spot we had flown over, killing my new aquaintance, who had been in the front seat where I had previously been. The AAIB report put the accident down to a loose article (torch battery) jamming the rudder during a botched stall turn / spin recovery. The loose article must already have been in the aircraft during my flight. Too close for comfort.