I've stepped into a covey of quail, or a nesting pheasant a couple of times. Thats a heart attack in the making.
Drew out on a bow hunt at Salt Plains once. Found where a brush pile had been pushed up when a new fence was getting put up.
It had a perfect spot in it that a guy could get into and still draw the bow. Walked in there during the night, and got in place. Just before daylight, noises kept coming from inside. Something moving.
Right at the crack of dawn, the noise came right by me, and left the brush pile. It was a big old fat rattler that had just eaten something.
Just knew his brother was going to be in there, so while almost shatting my pants, I bailed out, and left the area.