Hah!
I certainly don't think I have, but I've definitely seen a few things in the sky that I could not explain in terms of known astronomical phenomena or aerospace technology, and with other people around to verify what I was seeing. The last time it was with a friend of mine who's a physicist, and he specialized in astronomy back in school, and even with our mutual stockpile of geeky knowledge we couldn't figure out what the hell it could possibly be aside from a spacecraft or some sort of super-advanced, not-publicly-known method of flight. It was interesting when we first noticed it, but after watching it cartwheel around the sky for the better part of an hour and then seem to get closer as we were driving away, it scared the bejeezus out of me.
~Joe