Program: 2038
Project: 2038
Buggy source:
@Override public void onEntityDamage(EntityDamageEvent event){
  if (event instanceof EntityDamageByEntityEvent) {
    EntityDamageByEntityEvent e=(EntityDamageByEntityEvent)event;
    HumanNPC npc=NPCManager.getNPC(e.getEntity());
    if (e.getEntity() instanceof Player && e.getDamager() instanceof Player && npc != null) {
      e.setCancelled(true);
    }
    if (e.getEntity() instanceof Player && e.getDamager() instanceof Monster) {
      return;
    }
    if (npc.isHealer()) {
      Entity entity=e.getDamager();
      if (entity instanceof Player) {
        Player player=(Player)entity;
        int playerHealth=player.getHealth();
        int healerHealth=HealerPropertyPool.getStrength(npc.getUID());
        if (player.getItemInHand().getTypeId() == Citizens.healerTakeHealthItem) {
          if (playerHealth <= 19) {
            if (healerHealth >= 1) {
              player.setHealth(playerHealth + 1);
              HealerPropertyPool.saveStrength(npc.getUID(),healerHealth - 1);
              player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
            }
 else {
              player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
            }
          }
 else {
            player.sendMessage(ChatColor.GREEN + "String_Node_Str");
          }
        }
 else         if (player.getItemInHand().getTypeId() == Citizens.healerGiveHealthItem) {
          if (playerHealth >= 1) {
            if (healerHealth < HealerPropertyPool.getMaxStrength(npc.getUID())) {
              player.setHealth(playerHealth - 1);
              HealerPropertyPool.saveStrength(npc.getUID(),healerHealth + 1);
              player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
            }
 else {
              player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
            }
          }
 else {
            player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName()));
          }
        }
 else         if (player.getItemInHand().getType() == Material.DIAMOND_BLOCK) {
          if (healerHealth != HealerPropertyPool.getMaxStrength(npc.getUID())) {
            HealerPropertyPool.saveStrength(npc.getUID(),HealerPropertyPool.getMaxStrength(npc.getUID()));
            player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
            player.setItemInHand(null);
          }
 else {
            player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
          }
        }
      }
    }
  }
}

Fixed version (ground truth):
@Override public void onEntityDamage(EntityDamageEvent event){
  if (event instanceof EntityDamageByEntityEvent) {
    EntityDamageByEntityEvent e=(EntityDamageByEntityEvent)event;
    HumanNPC npc=NPCManager.getNPC(e.getEntity());
    if (e.getEntity() instanceof Player && e.getDamager() instanceof Player && npc != null) {
      e.setCancelled(true);
      if (npc.isHealer()) {
        Entity entity=e.getDamager();
        if (entity instanceof Player) {
          Player player=(Player)entity;
          int playerHealth=player.getHealth();
          int healerHealth=HealerPropertyPool.getStrength(npc.getUID());
          if (player.getItemInHand().getTypeId() == Citizens.healerTakeHealthItem) {
            if (playerHealth <= 19) {
              if (healerHealth >= 1) {
                player.setHealth(playerHealth + 1);
                HealerPropertyPool.saveStrength(npc.getUID(),healerHealth - 1);
                player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
              }
 else {
                player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
              }
            }
 else {
              player.sendMessage(ChatColor.GREEN + "String_Node_Str");
            }
          }
 else           if (player.getItemInHand().getTypeId() == Citizens.healerGiveHealthItem) {
            if (playerHealth >= 1) {
              if (healerHealth < HealerPropertyPool.getMaxStrength(npc.getUID())) {
                player.setHealth(playerHealth - 1);
                HealerPropertyPool.saveStrength(npc.getUID(),healerHealth + 1);
                player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
              }
 else {
                player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
              }
            }
 else {
              player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName()));
            }
          }
 else           if (player.getItemInHand().getType() == Material.DIAMOND_BLOCK) {
            if (healerHealth != HealerPropertyPool.getMaxStrength(npc.getUID())) {
              HealerPropertyPool.saveStrength(npc.getUID(),HealerPropertyPool.getMaxStrength(npc.getUID()));
              player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(npc.getStrippedName())+ "String_Node_Str");
              int x=player.getItemInHand().getAmount();
              ItemStack diamondBlock=new ItemStack(Material.DIAMOND_BLOCK,x - 1);
              player.setItemInHand(diamondBlock);
            }
 else {
              player.sendMessage(StringUtils.yellowify(npc.getStrippedName()) + "String_Node_Str");
            }
          }
        }
      }
    }
  }
}