This is going to sound a little lame, but I use to do a lot of solo backpacking and for about 6 months after I saw the Blair Witch Project I got a little wigged. The single worst actual experience was again on a solo 14er trip and in the middle of the night I kept hearing a thump, thump like something heavy was moving slowly towards me (I was thinking bear); I finally got the nerve to stick my head out with ice axe in hand only to find out as it was snowing and collecting on the pine trees then would drop after it got to a certain weight. All in all my own imagination is my worst enemy.