I've run into it too, though thankfully never run over it. The instances that I remember most happened on a paved trail around the local lake. The first was a F-250 maintanence truck trailing behind a jogger who was wearing headphones. The guy was completely oblivious. I had to ride up and tap him on the shoulder before he realized there was three tons of Detroit rolling iron crawling up his back.
The second involved a roller blader taking up the entire 20+ft width of the trail with his duck walk. Repeated "On your left!" and "On your right!" warning were completely ignored, and he only realized I was there when I road up about 3 ft behind him an yelled "pick a side!".
Whatever happened to "look both ways before crossing the street"?

We need a "<Bleeping> pay attention!!" Demotivational poster.