I have been hit twice while at anchor. Both other boats were also anchored but dragging. Neither time was there any damage or anything to get really excited about. I think my chain may have scratched the bottom of one of the boats a little. Both times it was the middle of the night. Both boats had small anchors and too little rode out. They gently drifted down on me and bumped me. I woke up, put out fenders and woke up the guys in the dragging boats.
When dragging boats bump into other anchored boats in fairly benign conditions, the likelihood of damage is small. The properly anchored boat (mine in these cases) just pushes out of the way.
If the conditions are more severe then an anchor watch is called for and would help to prevent getting hit.
Dragging boats normally drag fairly slowly.
Question - If a boat drags into your swinging circle in Texas and you feel there is a danger to property or life, can you shoot him?
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217