I've seen one. I was in a crop circle a few years back, lying on my back staring up at the clear blue sky. It was a hot, July day. This shimmering BOL like object moved across the sky, pretty high up, moving quite fast. It stopped suddently above me and my friend, paused for a few seconds then shot upwards. It didn't look like a bird or a weather balloon. It was weird. We both sat there, saying WTF??!!
It wasn't in my imagination. I had a friend with me. We both saw it and watched it together.
Footnote: I am not saying this was an alien spaceship or maker of crop circles. It was flying. We could not identify it. Therefore it was an unidentified flying object.

It is the only UFO I have ever seen.