I was on a subway train.  A man seated just behind me was making me feel uncomfortable so I turned around and looked over my left shoulder.  I saw a terrifying face and turned my head back to face straight ahead again.  Then, to confirm, I turned and looked again, and it was different, equally frightening face.  I turned away again.

This repeated as I looked back time and time again.   I saw a seemingly endless number of horrifying faces.