So, your brain can invent all kinds of stuff that seem way real. That's pretty powerful stuff, if you think about it. Your mind invented a reality in which you saw a big X where there wasn't really one there. It happened because of how we see the edges of things.
Edges are hard to recognize.
Little light sensors in our eyes shut down or boost their output to the brain when there's an edge. It usually helps make images more clear. But in our experiment, it created an image that isn't really there. I guess that makes it a bug in the wetware. Click the big question mark.