I tend to put a lot of this down to brain hiccups. Our brains are designed to make sense of things. When we see a person or object we imediately try to recognise and classify them. Once we determine who they are we decide how we are going to respond. We don't try to re-recognise them and occasionally get suprised if we started with the wrong recognition. So the brain hiccups or swallows down the wrong pipe. We get a memory shoved back into the visual processor instead of where its supposed to go. Suddenly, we realise there is something new to recognise, but it's already gone. Brain fishes around and trys to assign a value to it. And yes I have experienced it! When I was 4years old late in the afternoon I saw a teenage boy and girl walk arm in arm throught the wall of my bedroom as if up a flight of stairs. I sat up on the bed but they stayed in position. They took one, two, three steps with the color faiding and them growing darker with each one. By the third they had completely faded. Another time when I was high school age I was looking in the mirror of the bathroom and saw myself walk past the doorway behind me out of the corner of my eye. The doppelganger gave me an evil grin and stuck his tounge out. I even turned and went to the doorway, but no one was in the room. If any of these spoke, hung around, or ever seemed completely solid for a fraction of a second more I would believe something else. Theres just always room for doubt.