I had a very unnerving experience on a flight over Portugal. We were at around FL33 when looking out of the starboard window, I saw another aircraft approaching us in a curving trajectory. It actually passed directly under our aircraft and I remember distinctly being able to see clearly the faces of the pilots.
Sadly my camera was in my bag in the overhead locker so I was unable to record the moment. I guess the normal parameters of separation were being observed, but there was definitely a frisson sweeping up my back as the two aircraft converged.