This one happened to my neighbours, not to me, but they told my mother about it and showed her the picture:
My neighbours went for a weekend break at a historic house a few hours away. They were given a gorgeous room and were so thrilled with it that they took several photos, including one of the wife sitting on the ornate four poster bed.
This was before almost everyone had digital cameras, so my neighbours took their cameras film to be developed, and when they got it back, they noticed that in the photo of the wife sitting on the bed, there was a grey face floating at the top of the photo, high above the bed.
They originally thought the face had come from another photo, but when they examined all the photos, they couldn't find any that the face could have come from, so they got a friend to check the negative, and sure enough, the face was on the negative as well!