These hits do exist and they don't just do extra damage, when landed these hits can cause a level 1 goblin to take down a thane over a thousand times larger than it. From my experience you can eliminate crits completely by taking some time to make sure you are fully equipped. Every thane has 1 head, 1 body, 2 arms(some thanes may have more). I find that crits only seem to happen when one of these equip slots are open, if you equip every slot with an item you make crits extremely rare or nonexistant. Before any fight make sure you are fully equipped.