In 1994 I was riding a trail up Mt. Spokane with a friend. We hadn't gone too far. I was coming up on a sweeper corner that people would take pretty fast if they were coming down, so I:
1. watched for oncoming riders
2. listened for oncoming riders
3. yelled around the corner to alert any hypothetical oncoming riders
4. had my escape route planned
5. and then I was being loaded onto a gurney with a big old collar thing around my neck, after being run into head-on by an oncoming rider and evidently taking a direct forehead-to-forehead hit, separated by two helmet shells. There's about an hour missing from my memory banks from the concussion, I was in a fog for days.
Pro tip: keep your helmet on, even when climbing at low speed in a quiet forest.