Program: 58094
Project: 58094
Buggy source:
@Override public boolean execute(final Player plr,final String... args){
  if (args.length < 2) {
    PlayerFunctions.sendMessage(plr,C.HELPER_NEED_ARGUMENT);
    return true;
  }
  if (!PlayerFunctions.isInPlot(plr)) {
    PlayerFunctions.sendMessage(plr,C.NOT_IN_PLOT);
    return true;
  }
  final Plot plot=PlayerFunctions.getCurrentPlot(plr);
  if ((plot.owner == null) || !plot.hasRights(plr)) {
    PlayerFunctions.sendMessage(plr,C.NO_PLOT_PERMS);
    return true;
  }
  if (args[0].equalsIgnoreCase("String_Node_Str")) {
    UUID uuid;
    if (args[1].equalsIgnoreCase("String_Node_Str")) {
      uuid=DBFunc.everyone;
    }
 else {
      uuid=UUIDHandler.getUUID(args[1]);
    }
    if (!plot.helpers.contains(uuid)) {
      if (plot.owner == uuid) {
        PlayerFunctions.sendMessage(plr,C.ALREADY_OWNER);
        return false;
      }
      if (plot.trusted.contains(uuid)) {
        plot.trusted.remove(uuid);
        DBFunc.removeTrusted(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(uuid));
      }
      if (plot.denied.contains(uuid)) {
        plot.denied.remove(uuid);
        DBFunc.removeDenied(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(uuid));
      }
      plot.addHelper(uuid);
      DBFunc.setHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
      final PlayerPlotHelperEvent event=new PlayerPlotHelperEvent(plr,plot,uuid,true);
      Bukkit.getPluginManager().callEvent(event);
    }
 else {
      PlayerFunctions.sendMessage(plr,C.ALREADY_ADDED);
      return false;
    }
    PlayerFunctions.sendMessage(plr,C.HELPER_ADDED);
    return true;
  }
 else   if (args[0].equalsIgnoreCase("String_Node_Str")) {
    if (args[1].equalsIgnoreCase("String_Node_Str")) {
      final UUID uuid=DBFunc.everyone;
      if (!plot.helpers.contains(uuid)) {
        PlayerFunctions.sendMessage(plr,C.WAS_NOT_ADDED);
        return true;
      }
      plot.removeHelper(uuid);
      DBFunc.removeHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
      PlayerFunctions.sendMessage(plr,C.HELPER_REMOVED);
      return true;
    }
    final UUID uuid=UUIDHandler.getUUID(args[1]);
    plot.removeHelper(uuid);
    DBFunc.removeHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
    final PlayerPlotHelperEvent event=new PlayerPlotHelperEvent(plr,plot,uuid,false);
    Bukkit.getPluginManager().callEvent(event);
    PlayerFunctions.sendMessage(plr,C.HELPER_REMOVED);
  }
 else {
    PlayerFunctions.sendMessage(plr,C.HELPER_NEED_ARGUMENT);
    return true;
  }
  return true;
}

Fixed version (ground truth):
@Override public boolean execute(final Player plr,final String... args){
  if (args.length < 2) {
    PlayerFunctions.sendMessage(plr,C.HELPER_NEED_ARGUMENT);
    return true;
  }
  if (!PlayerFunctions.isInPlot(plr)) {
    PlayerFunctions.sendMessage(plr,C.NOT_IN_PLOT);
    return true;
  }
  final Plot plot=PlayerFunctions.getCurrentPlot(plr);
  if ((plot.owner == null) || !plot.getOwner().equals(plr)) {
    PlayerFunctions.sendMessage(plr,C.NO_PLOT_PERMS);
    return true;
  }
  if (args[0].equalsIgnoreCase("String_Node_Str")) {
    UUID uuid;
    if (args[1].equalsIgnoreCase("String_Node_Str")) {
      uuid=DBFunc.everyone;
    }
 else {
      uuid=UUIDHandler.getUUID(args[1]);
    }
    if (!plot.helpers.contains(uuid)) {
      if (plot.owner == uuid) {
        PlayerFunctions.sendMessage(plr,C.ALREADY_OWNER);
        return false;
      }
      if (plot.trusted.contains(uuid)) {
        plot.trusted.remove(uuid);
        DBFunc.removeTrusted(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(uuid));
      }
      if (plot.denied.contains(uuid)) {
        plot.denied.remove(uuid);
        DBFunc.removeDenied(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(uuid));
      }
      plot.addHelper(uuid);
      DBFunc.setHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
      final PlayerPlotHelperEvent event=new PlayerPlotHelperEvent(plr,plot,uuid,true);
      Bukkit.getPluginManager().callEvent(event);
    }
 else {
      PlayerFunctions.sendMessage(plr,C.ALREADY_ADDED);
      return false;
    }
    PlayerFunctions.sendMessage(plr,C.HELPER_ADDED);
    return true;
  }
 else   if (args[0].equalsIgnoreCase("String_Node_Str")) {
    if (args[1].equalsIgnoreCase("String_Node_Str")) {
      final UUID uuid=DBFunc.everyone;
      if (!plot.helpers.contains(uuid)) {
        PlayerFunctions.sendMessage(plr,C.WAS_NOT_ADDED);
        return true;
      }
      plot.removeHelper(uuid);
      DBFunc.removeHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
      PlayerFunctions.sendMessage(plr,C.HELPER_REMOVED);
      return true;
    }
    final UUID uuid=UUIDHandler.getUUID(args[1]);
    plot.removeHelper(uuid);
    DBFunc.removeHelper(plr.getWorld().getName(),plot,Bukkit.getOfflinePlayer(args[1]));
    final PlayerPlotHelperEvent event=new PlayerPlotHelperEvent(plr,plot,uuid,false);
    Bukkit.getPluginManager().callEvent(event);
    PlayerFunctions.sendMessage(plr,C.HELPER_REMOVED);
  }
 else {
    PlayerFunctions.sendMessage(plr,C.HELPER_NEED_ARGUMENT);
    return true;
  }
  return true;
}