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.

A big +1. Where I hunt there are some big ole spiders, and they really freak me out. I always walk with my bow in front of me in a swinging motion to knock down any webs.

I was in my stand and a neighbors pitbull followed my scent trail and sat at the base of my tree snarling and barking until he finally left an hour later. I thought that i was gonna have to shoot him.

The most scared i ever remember being was when i was checking my deer camera last fall. I was walking down this deer trail and i happened to look down and see a hole in the ground filled with wasps. Needless to say i took off running (I was only wearing athletic shorts).

Sitting in my stand that was placed in a nice thick funnel in a NH State Forest and I hear someone shooting a .22 caliber, maybe 75 yards from me.
Next thing you know I hear a much larger caliber go off and figured I'd yell out, as this would do more than just hurt me, probably kill me!
So I screamed out "what the **** are you doing". They replied "are you hunting", of course I said yes, and they said "sorry" and they packed up and left.
I took my stand down and never returned.
Bullets wizzing past my head kind of soured me![>:]