(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'm not suggesting it was a hallucination but that you could have both seen an illusion created by the light and shadows. If the light didn't change then perhaps it disappeared as your eyes adjusted. There are optical illusions where you can see a person one second then see a different figure and lose the person and have difficulty seeing it the first way again. Or it could have been a ghost for all I know.