Type of Bug/Exploit:
The spell Stoneskin seems to be blocking up to 15 points of damage (and possibly more) at a time. If it's not supposed to block this much, it should probably be fixed. This is true of both NPCs and PCs, from what I can tell.
Area of Bug Exploit All of Ysgard (Haven't tested Hala)
NPCs involved Elf Mercenary (staircase of ysgard) is the only NPC I have seen cast the spell.
Chests involved Nope.
Timestamp on when it happened Yesterday and the day before this....I don't know the exact times, I am sorry.
Frequency of occurance. Whenever Stoneskin is cast.