Program: 163915
Project: 163915
Buggy source:
@Override public void activate(){
  Char target=Actor.findChar(pos);
  if (target == null) {
    for (    Char ch : Actor.chars()) {
      Ballistica bolt=new Ballistica(pos,ch.pos,Ballistica.PROJECTILE);
      if (bolt.collisionPos == ch.pos && (target == null || Dungeon.level.distance(pos,ch.pos) < Dungeon.level.distance(pos,target.pos))) {
        target=ch;
      }
    }
  }
  if (target != null) {
    final Char finalTarget=target;
    final GrimTrap trap=this;
    ((MagicMissile)target.sprite.parent.recycle(MagicMissile.class)).reset(MagicMissile.SHADOW,DungeonTilemap.tileCenterToWorld(pos),target.sprite.center(),new Callback(){
      @Override public void call(){
        if (!finalTarget.isAlive())         return;
        if (finalTarget == Dungeon.hero) {
          if (((float)finalTarget.HP / finalTarget.HT) >= 0.9f) {
            finalTarget.damage((finalTarget.HP - 1),trap);
          }
 else {
            finalTarget.damage(finalTarget.HP,trap);
          }
          Sample.INSTANCE.play(Assets.SND_CURSED);
          if (!finalTarget.isAlive()) {
            Dungeon.fail(GrimTrap.class);
            GLog.n(Messages.get(GrimTrap.class,"String_Node_Str"));
          }
        }
 else {
          finalTarget.damage(finalTarget.HP,this);
          Sample.INSTANCE.play(Assets.SND_BURNING);
        }
        finalTarget.sprite.emitter().burst(ShadowParticle.UP,10);
        if (!finalTarget.isAlive())         finalTarget.next();
      }
    }
);
  }
 else {
    CellEmitter.get(pos).burst(ShadowParticle.UP,10);
    Sample.INSTANCE.play(Assets.SND_BURNING);
  }
}

Fixed version (ground truth):
@Override public void activate(){
  Char target=Actor.findChar(pos);
  if (target == null) {
    for (    Char ch : Actor.chars()) {
      Ballistica bolt=new Ballistica(pos,ch.pos,Ballistica.PROJECTILE);
      if (bolt.collisionPos == ch.pos && (target == null || Dungeon.level.distance(pos,ch.pos) < Dungeon.level.distance(pos,target.pos))) {
        target=ch;
      }
    }
  }
  if (target != null) {
    final Char finalTarget=target;
    final GrimTrap trap=this;
    int damage;
    if (finalTarget == Dungeon.hero && ((float)finalTarget.HP / finalTarget.HT) >= 0.9f) {
      damage=finalTarget.HP - 1;
    }
 else {
      damage=finalTarget.HP;
    }
    final int finalDmg=damage;
    Actor.add(new Actor(){
{
        actPriority=Integer.MIN_VALUE;
      }
      @Override protected boolean act(){
        final Actor toRemove=this;
        ((MagicMissile)finalTarget.sprite.parent.recycle(MagicMissile.class)).reset(MagicMissile.SHADOW,DungeonTilemap.tileCenterToWorld(pos),finalTarget.sprite.center(),new Callback(){
          @Override public void call(){
            finalTarget.damage(finalDmg,trap);
            if (finalTarget == Dungeon.hero) {
              Sample.INSTANCE.play(Assets.SND_CURSED);
              if (!finalTarget.isAlive()) {
                Dungeon.fail(GrimTrap.class);
                GLog.n(Messages.get(GrimTrap.class,"String_Node_Str"));
              }
            }
 else {
              Sample.INSTANCE.play(Assets.SND_BURNING);
            }
            finalTarget.sprite.emitter().burst(ShadowParticle.UP,10);
            Actor.remove(toRemove);
            next();
          }
        }
);
        return false;
      }
    }
);
  }
 else {
    CellEmitter.get(pos).burst(ShadowParticle.UP,10);
    Sample.INSTANCE.play(Assets.SND_BURNING);
  }
}