Paul Freeman of South Wales created a fictional film and uploaded it to YouTube. When he viewed the screen shot (you can view it here: Critic Monkey), it looked to him like someone was peering over his shoulder:
"I recently made a short fictional film for a competition running on YouTube. When viewing the still image (before the video is played) randomly selected by YouTube for my channel (youtube.com/criticmonkey), I noticed a demonic-looking face top left of my head in the mirror."
A new urban legend? Interesting face-like image appears in screenshot. This was not a stunt by Paul, just something that showed up of interest. It could have been caused by pixellation. When we watched Paul's video, we could not see it appear in the mirror like it does here. Ghost? You decide...