Once, while still a teenager suffering from typical teenage acne, I chanced to sleep for 14 hours straight one weekend. This was not uncommon for me, and, unfortunately, it would occasionally cause ingrown hairs.
Usually, hairs become ingrown due to irritation of a follicle whose hair is so short it does not extend out of the skin. If you're in the practice of waxing your body hair, you've probably had this happen.
I'm pretty sure that, somehow, sleeping for extended periods of time did this to me. Perhaps the hairs were all brand new, and that's why it doesn't happen any more -- I'm already as well-upholstered as I'm going to get, so there are no new hairs springing up deep within my hide.
The worst thing that ever happened in my long history of dermatological distress was the time I had an ingrown nostril hair poke out the other side. I suspect that what happened was this: I had a pimple starting on the outside of my nose and an ingrown hair on the inside. They were on the same axis and the hair managed to turn around 180 degrees instead of just pushing on the skin that had grown over it. Anyway, I popped this pimple, and when I had mopped away the blood, I found a tiny black dot in the center of the mess. I pushed on my nostril from the inside, and the dot proved itself to be a hair. It extended almost an eighth of an inch. Presently, I got over my shock and extracted it with tweezers. The overall length was about five sixteenths of an inch. The root did not come out with it, so it could have been broken off from an even longer hair. Last week, I got one that was at least half an inch.