I've never dented a barrel (knock on wood) that I recall, but I've taken out what seems like hundreds of dents, including a couple that a dealer put in a gun between the time I paid for it and when I picked it up.
My guess is that people turn quickly and bang the barrel into anything with a sharp corner, as this seems to be what I've taken out most. Other types of dents I've seen were from standing them on the butt on the ground and falling over off the side of a car/truck onto rocks/pavement. Those types of dents usually have a sharp point from a rock at the bottom of the dent, that won't come out without filing down.
