Health bars aren't always properly updated in battle. Two examples:
1) When an enemy unit takes damage from attacking your unit with a shrill shield, the enemy's health bar isn't updated.
2) When you heal via drinking a health potion, the health bar isn't updated until your next turn.
3) When a troll shaman casts barbs, it only updates the shaman's health bar, not the health bar of the opposing unit it damages.
4) Double strike doesn't update health bars after the second hit.