This probably happens all the time, but I've never read about it here.
Last night I had a flat tire on my way home. When I examined the tire, I found a sharp object embedded in it. When I dug it out, I was surprised to discover that it was a wood splinter. Looking further, I discovered two more splinters that had also penetrated the tire. I had been riding on our town's Rivertrail just before I lost pressure. There are many board walks and bridges on this trail, all made of wood. Evidently one of these rough, weathered wooden sections had given me three flats for the price of one. Of course I've been riding on the trail for years and this was the first time I had problem.
I guess I learned two lessons. The first was to watch the wood more carefully when I ride. The second was that if you find the object that caused a flat--keep looking! There might be more sharpies still embedded in the tire.