I got all the way to the far west side of Denver before I realized that I didn't have my helmet in the truck with me. I had my mountain bike, and my shoes and gloves. Just no helmet...
I gave it some thought, and finally decided that I could probably get away with one short mountain bike ride without a helmet. But, I wasn't really comfortable with it.
I ride my road bike, routinely, with no helmet. I have done more than one 100 mile loop with only a cap on my head. But, I had never gone mountain biking without a helmet, before. I told myself it would be okay, so long as I took it easy and just rode to be out in the woods, rather than for speed.
I got all the way to the trail head, telling myself it would be okay. Then, I turned around and drove to a bike shop in Lakewood and bought a helmet.
I enjoyed the ride, a lot, after that.