Speaking of nature, one unforgettable experience I once had was walking in a park, it was twilight and an owl swooped down from a tree right in front of me and grabbed a squirrel, interesting experience.
Doing a lot of hunting in my younger years, I saw similar happenings numerous times by owls, hawks, etc. Had a 10 foot or so shark take a good sized Speckled Trout just as I got him to the boat offshore of Galveston.
But the unexpected thing that scared the bejesus out of me the most was once in Spring of 1967 when I was driving from Mexia, Texas, north to Corsicana and just as I came over a hill, lighting up a cigarette and looked up to find a B-52 flying nap of the earth coming directly at me at what looked to be 10 feet off the ground-- I damn near crapped my pants! I did have to pull off the road and calm down afterward.
I still occasionally dream about that.