A couple of times, many years ago. The first one was when I was 13 or 14, in middle school (I am from Italy). Last hour of lesson, very "interesting" as I was absolutely looking of the window. All of a sudden, a peculiar "craft" catches my attention; Imagine it as a teardrop-shaped orange blob, with two similarly shaped "pods" attached at each side of it, at the bottom. The general shape & size was that on an helicopter, but there was no sound nor any visible rotors. It also flew more like a plane - straight, stable and very fast. One of my classmates blurted, mid lesson, "what the hell is that thing?"
Never saw any pictures of anything even similar, might very well have been some RC toy (although it was 1992-1993 and they weren't that popular) or an old light plane (there were some made in odd shapes in the past).
Second one, it was 1999. August evening, I was hanging out with some friends. All of a sudden one of them goes "hey, is that an UFO?". We all laughed at him because he had a reputation as a prankster. He kept insisting that we took a look, that he wasn't kidding - and he looked scared. So we do, and there is indeed a bright light in the sky, just visible off the top of a building. "Helicopter" we dismiss it, but it starts moving towards the right of our FOV - wavy and very fast movement - only to come to a sudden halt. Then it struck us that we couldn't hear a sound - it was no chopper. It finally started moving away from where we were, I tried to get my friends to go in the street so we could see better, but then I realized they were all absolutely taken over by fright. The object accelerated to a what looked like a very high speed and disappeared in the distance.
Now, back then there was the Balcan crisis and, being in Naples, I was used at seeing all types of military craft movements - including a spectacular formation of 15 Apache Gunships flying over the city and to the airport. The object I saw could have been an experimental aircraft - however, are we so sure that the concept of an experimental, gravity defying, physics bending military aircraft is any more believable than the "not terrestrial" explanation?