Apparently negative heal amp is multiplying the negative damage that you take even if you wouldn't heal from the negative damage. This can be problematic as some items give multiple kinds of heal amp.
Bring Darkness from Shadowdancer doesn't seem to be applying its untyped damage on harmful spellcast if you have First Blood. At least on alchemist, I haven't tested with other classes yet.
Edit: The Bring Darkness bug actually appears to happen because it doesn't test if the mob is above 90% health until after your first spell lands. So you cast spell on an uninjured mob, spell does some damage, knocks mob down to 89% health, it sees its below the threshold so it doesn't give the extra effect intended.
Also Hellball doesn't seem to be working correctly. Damage dice doesn't line up with the tooltip. Either the tooltip is wrong or the spell is doing the wrong damage.