I´ve had a similar thing happen to me - while I was in the fight inside the temple of Selûne, where Gut the priestess is - I apparently rolled a natural 20 ( critical hit ) and the boss goblin retaliated, my damage was negated, as in did 0 damage aswell as the goblin´s retaliation, that is to say... There where no numbers up on the screen just me swinging him for a critical and he retaliating animation but we where both at full health still after the round - this spun into a huge bug fest after that, as Gut cast a telekinetic blast on one of the statues that crumbled ontop of 2 of my characters and 3 goblins, that resulted in thouse 5 characters not being able to attack for the remainder of the encounter until I gave up and loaded a saved game 5 minutes prior to that fight.
