The Bright Shield Rune's final spell, Forgiver Sign, heals the entire party up to 2,000 hit points of damage. The remaining value from the pool is then used to do damage to one enemy. So the spell can be used with an entirely healthy party to cause ~2,000 hit points of damage to an enemy, depending on enemy resistance and the Hero's affinity with the Rune. If your party needs 900 points of healing, 1,100 HPs would be used as base damage to one enemy.
There is a bug that arises when the party requires more than 2,000 HP of healing. In that case, there are no HPs left to do damage, and the targeted enemy appears to receive massive damage (the number is usually white, indicating harm), or a massive heal. In either case, an extremely large number is overlaid, and the enemy does not die.
Affected Versions
All versions are affected by this bug.
Cause
In reality, the enemy is neither harmed nor healed in this case. When Forgiver Sign's pool of HP is depleted in the healing phase, the game sets the damage to -1. With this damage value, the enemy's hit points will not be updated, nor will the value be copied to the display buffer. However, the game still needs to display something for damage at the end of the animation, and it winds up retrieving garbage data.
What this means is the bug is entirely cosmetic. If you use Forgiver Sign against the Beast Rune's head, and an 18,376 point heal pops up, you haven't actually undone all the damage you've dealt up to that point.
Fix
Modify the
/CDROM/150_BPRG/BUFF0/BP0_FST.BIN and
/CDROM/150_BPRG/BUFF0/BP0_SEC.BIN modules to set the damage to 0 instead of -1. The damage routines will be executed, but will do nothing, and the display buffer will be updated with a zero value.