Your rifle, bullets, and your tag.
I remember one pronghorn antelope hunt that I went on where I got 20 miles from home and remembered that I didn't bring my .257 AI ammo. I got to my hunting area about an hour later than I had planned.
When we lived in western Colorado my wife and I both drew antelope tags in eastern Colorado. It wasn't until we got to our hunting area that she discovered that she had left her tag at home.
Not exactly hunting, but one winter when I lived in NW Montana a friend and I went to Lake Koocanusa, west of Eureka, ice fishing. When we got to the lake, I had my .30-06 and bullets in case we saw any coyotes, but I had forgot my fishing rods. We didn't catch any fish or see any coyotes.
