(Thread prompted by the one about seeing someone with lizard eyes, who is possessed by a demon)
I have once. Waken from a dead cold sleep, my girlfriend and future wife said to me "we're not alone". I looked at the foot of the bed and their was a very angry young boy. I was shocked and became afraid myself as I sensed a power there, and then, like a balloon popping, he vanished.
She asked me what I saw, I said a very angry young boy, who looked to be age 9-10. She confirmed that was what she saw too.
I suppose it could have been an angry guardian angel, or even a passing demon, but let's go with ghost.
I have never doubted things spiritual since that day. I was about age 19.
I've seen a lifetime of weird things. New England was a hotbed. One time I was working on a farm that dated back to the 1600s. I was in the middle of a pasture kneeled over connecting hose or something, working alone. I glanced up and saw a pair of legs from the waist down walk past me- grey and opaque like a cloud. It looked like the shape of a pair of trousers and boots and took about 5-6 steps. Then they just faded away.
This actually sounds badass. Kinda like you were working in a field and overlapped some part of the space-time continuum. Both of you working in the same space. Just different times. A glitch in the matrix.