Program: 1890
Project: 1890
Buggy source:
@Override public boolean onCommand(CommandSender sender,Command command,String commandLabel,String[] args){
  if (!(sender instanceof Player)) {
    sender.sendMessage(MessageUtils.mustBeIngameMessage);
    return true;
  }
  Player player=(Player)sender;
  HumanNPC npc=null;
  boolean returnval=false;
  if (NPCManager.validateSelected((Player)sender)) {
    npc=NPCManager.get(NPCManager.selectedNPCs.get(player.getName()));
  }
 else {
    player.sendMessage(MessageUtils.mustHaveNPCSelectedMessage);
    return true;
  }
  if (!npc.isTrader()) {
    player.sendMessage(ChatColor.RED + "String_Node_Str");
    return true;
  }
  if (args[0].equalsIgnoreCase("String_Node_Str")) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      HelpUtils.sendTraderHelp(sender);
    }
 else {
      sender.sendMessage(MessageUtils.noPermissionsMessage);
    }
    return true;
  }
 else   if (args.length >= 2 && args[0].contains("String_Node_Str") && (args[1].contains("String_Node_Str") || args[1].contains("String_Node_Str"))) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      displayList(player,npc,args,args[1].contains("String_Node_Str"));
    }
 else {
      player.sendMessage(MessageUtils.noPermissionsMessage);
    }
    returnval=true;
  }
 else   if (args.length == 1 && args[0].contains("String_Node_Str")) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      if (!EconomyHandler.useIconomy())       player.sendMessage(MessageUtils.noEconomyMessage);
 else       displayMoney(player,npc);
    }
 else     player.sendMessage(MessageUtils.noPermissionsMessage);
    returnval=true;
  }
  if (!NPCManager.validateOwnership(player,npc.getUID())) {
    if (!returnval) {
      player.sendMessage(MessageUtils.notOwnerMessage);
    }
    return true;
  }
 else {
    if (args.length == 3 && args[0].equalsIgnoreCase("String_Node_Str")) {
      if (Permission.canModify(player,npc,"String_Node_Str")) {
        if (!EconomyHandler.useIconomy())         player.sendMessage(MessageUtils.noEconomyMessage);
 else         changeBalance(player,npc,args);
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
 else     if (args.length == 3 && (args[0].contains("String_Node_Str") || args[0].contains("String_Node_Str"))) {
      if (Permission.canModify(player,npc,"String_Node_Str")) {
        changeTraderStock(player,npc,args[1],args[2],args[0].contains("String_Node_Str"));
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
 else     if (args.length == 2 && (args[0].contains("String_Node_Str"))) {
      if (Permission.isAdmin(player)) {
        changeUnlimited(npc,sender,args[1]);
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
    PropertyManager.save(npc);
  }
  return returnval;
}

Fixed version (ground truth):
@Override public boolean onCommand(CommandSender sender,Command command,String commandLabel,String[] args){
  if (!(sender instanceof Player)) {
    sender.sendMessage(MessageUtils.mustBeIngameMessage);
    return true;
  }
  Player player=(Player)sender;
  HumanNPC npc=null;
  boolean returnval=false;
  if (NPCManager.validateSelected((Player)sender)) {
    npc=NPCManager.get(NPCManager.selectedNPCs.get(player.getName()));
  }
 else {
    player.sendMessage(MessageUtils.mustHaveNPCSelectedMessage);
    return true;
  }
  if (!npc.isTrader()) {
    player.sendMessage(ChatColor.RED + "String_Node_Str");
    return true;
  }
  if (args[0].equalsIgnoreCase("String_Node_Str")) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      HelpUtils.sendTraderHelp(sender);
    }
 else {
      sender.sendMessage(MessageUtils.noPermissionsMessage);
    }
    return true;
  }
 else   if (args.length >= 2 && args[0].contains("String_Node_Str") && (args[1].contains("String_Node_Str") || args[1].contains("String_Node_Str"))) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      displayList(player,npc,args,args[1].contains("String_Node_Str"));
    }
 else {
      player.sendMessage(MessageUtils.noPermissionsMessage);
    }
    returnval=true;
  }
 else   if (args.length == 1 && args[0].contains("String_Node_Str")) {
    if (Permission.canUse(player,npc,"String_Node_Str")) {
      if (!EconomyHandler.useIconomy())       player.sendMessage(MessageUtils.noEconomyMessage);
 else       displayMoney(player,npc);
    }
 else     player.sendMessage(MessageUtils.noPermissionsMessage);
    returnval=true;
  }
  if (!NPCManager.validateOwnership(player,npc.getUID())) {
    if (!returnval) {
      player.sendMessage(MessageUtils.notOwnerMessage);
    }
    return true;
  }
 else {
    if (args.length == 3 && args[0].contains("String_Node_Str")) {
      if (Permission.canModify(player,npc,"String_Node_Str")) {
        if (!EconomyHandler.useIconomy())         player.sendMessage(MessageUtils.noEconomyMessage);
 else         changeBalance(player,npc,args);
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
 else     if (args.length == 3 && (args[0].contains("String_Node_Str") || args[0].contains("String_Node_Str"))) {
      if (Permission.canModify(player,npc,"String_Node_Str")) {
        changeTraderStock(player,npc,args[1],args[2],args[0].contains("String_Node_Str"));
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
 else     if (args.length == 2 && (args[0].contains("String_Node_Str"))) {
      if (Permission.isAdmin(player)) {
        changeUnlimited(npc,sender,args[1]);
      }
 else {
        player.sendMessage(MessageUtils.noPermissionsMessage);
      }
      returnval=true;
    }
    PropertyManager.save(npc);
  }
  return returnval;
}