A good few years ago, pre 9-11, when jump seats were not an issue, I was travelling DUB-BRU, and we heard a call from ATC, Shamrock xxx, opposite direction traffic 10 miles at FL280, with us at the time at 270. Call was acknowledged, and 3 pairs of eyes went out of the window. It didn't take long to spot, it was reciprocal heading, 1000 Ft above us, closing speed was of course massive, and in a few seconds, it went directly over us. We commented at the time that we were happy that ATC had told us about it, as at that level and speed, (and this was pre full TCAS implementation), it was not immediately clear if it was above us, and as it went past, 1000 Ft looked like a very small gap indeed.
I've sat in the hold at Bovingdon more than a few times (again on the jump seat) and on a clear day, it's amazing to look out of the windows at more than a few aircraft all flying round one above the other, and I can well see how some people would be concerned at seeing another aircraft so close.
I've also seen the rivets down the side of a Dan Air 1-11, we were doing VOR radial tracking training with a full radar service from Dunsfold at 2400 Ft over Midhurst, and the LGW controller had suppressed all non LGW returns, and positioned the 1-11 over Midhurst on the base of the TMA at 2500 Ft to allow it to position from Lasham back to LGW.
That was a hell of a sight closer that we were comfortable with, and to be fair, the Dunsfold controller that was giving us a radar service at the time filed the airmiss for us. He was less than impressed too, as it happened very fast, though he was able to give us about 15 seconds warning of "opposite direction traffic, level unknown, not working with me", so I'm guessing that the 1-11 didn't have mode C on at the time.
It was another of those IFR sorts of days, where we were in and out of the layers, ideal for IFR training, but worrying when something that size pops out of a cloud almost on top of you
(And in passing, what is it about VOR's that they seem to act like magnets to the largest and nastiest CB's within 100 Miles? The number of times I had to go round rather than go through at places like BCN & STU were legion
)
Then there was the Cherokee from Belgium to Biggen that blasted across Lydd's procedural approach path at 1500 Ft on a very iffy day when we were doing ILS training, it popped out of a cloud 200 Mtrs in front of us, and went into another almost immediately. Again, we'd been sort of warned, Kent Radar had called Lydd to tell them this eeejit was about to blast through, so Lydd were just telling us about it when we saw it!