My character has 47 HP and no fire resistance. I created the following spell:
Fire Damage 50 pts on Self
Restore Health 50 pts on Self

Casting this spell did not kill me. I would have expected the Fire Damage to be fatal, but it turns out that the Restore Health canceled it out.

Well yeah if they would be cast at exactly the same time, I guess it should. Good coding on Bethesda's part (at least, in this case).

What happens if your character is affected by fire weakness? :)