This happened when I was a kid, about 13 years old. I was home alone on a Sunday evening (my parents and sister were out). I was bored, so being the nosy little kid that I was, I decided to get the binoculars, turn out all the lights and spy into the windows of the houses across the street from me. Pretty pointless exercise in truth, as it was dark outside, and most of the curtains of the other houses were drawn.
I was crouched down at the bedroom window, when I spotted a guy walking down the street. He was walking really slowly, so I focused in on him and he was having a right good old chin wag to himself. Then he stopped dead in his tracks and just stood there, right across the street from my house. And he started laughing to himself. I gave a little chuckle to myself at this point, safe in the knowledge that there could be no way that this guy could even know he was being watched (all the lights in my room were out). But then, he turned around so that he was facing my house, and looked up right into my window, and stared right at me. Then he pointed right at me, and made a "slit-throat" gesture towards me.
Needless to say, I almost *beep* myself. I jumped right back and ran downstairs. I sat there for a couple of minutes, feeling totally freaked out. Then, the front door to my house opened. I ran straight back upstairs, at a speed that I reckon would have done Ben Johnson on drugs proud during the 100 metres, and locked myself in the bathroom!
Next thing I heard were my parents and sister pissing themselves laughing at the sight of me legging it upstairs while screaming like a little pansy!!! I learned my lesson.