Program: 1868
Project: 1868
Buggy source:
/** 
 * Purchase a teleport
 * @param player
 * @param wizard
 * @param op
 */
public static void buy(Player player,HumanNPC npc,Operation op){
  if (!EconomyHandler.useEconomy() || EconomyHandler.canBuy(op,player)) {
    if (EconomyHandler.useEconomy()) {
      boolean canSend=false;
      WizardNPC wizard=npc.getToggleable("String_Node_Str");
      double paid=EconomyHandler.pay(op,player);
      if (paid > 0) {
        String msg=ChatColor.GREEN + "String_Node_Str" + StringUtils.wrap(EconomyHandler.getPaymentType(op,"String_Node_Str" + paid));
switch (op) {
case WIZARD_TELEPORT:
          msg+="String_Node_Str" + StringUtils.wrap(wizard.getCurrentLocationName()) + "String_Node_Str";
        if (teleportPlayer(player,npc)) {
          canSend=true;
        }
      break;
case WIZARD_SPAWNMOB:
    msg+="String_Node_Str" + StringUtils.wrap(wizard.getMob().name().toLowerCase().replace("String_Node_Str","String_Node_Str")) + "String_Node_Str";
  if (spawnMob(player,npc)) {
    canSend=true;
  }
break;
case WIZARD_CHANGETIME:
msg+="String_Node_Str" + StringUtils.wrap(wizard.getTime()) + "String_Node_Str";
if (changeTime(player,npc)) {
canSend=true;
}
break;
case WIZARD_TOGGLESTORM:
msg+="String_Node_Str" + StringUtils.wrap(player.getWorld().getName()) + "String_Node_Str";
if (toggleStorm(player,npc)) {
canSend=true;
}
break;
default :
msg=ChatColor.RED + "String_Node_Str";
canSend=true;
break;
}
if (canSend) {
player.sendMessage(msg);
}
}
}
 else {
player.sendMessage(ChatColor.GRAY + "String_Node_Str");
}
}
 else if (EconomyHandler.useEconomy()) {
player.sendMessage(MessageUtils.getNoMoneyMessage(op,player));
}
}

Fixed version (ground truth):
/** 
 * Purchase a teleport
 * @param player
 * @param wizard
 * @param op
 */
public static void buy(Player player,HumanNPC npc,Operation op){
  if (!EconomyHandler.useEconomy() || EconomyHandler.canBuy(op,player)) {
    if (EconomyHandler.useEconomy()) {
      boolean canSend=false;
      WizardNPC wizard=npc.getToggleable("String_Node_Str");
      double paid=EconomyHandler.pay(op,player);
      if (paid > 0 || EconomyHandler.isFree(player,op)) {
        String msg=ChatColor.GREEN + "String_Node_Str" + StringUtils.wrap(EconomyHandler.getPaymentType(op,"String_Node_Str" + paid));
switch (op) {
case WIZARD_TELEPORT:
          msg+="String_Node_Str" + StringUtils.wrap(wizard.getCurrentLocationName()) + "String_Node_Str";
        if (teleportPlayer(player,npc)) {
          canSend=true;
        }
      break;
case WIZARD_SPAWNMOB:
    msg+="String_Node_Str" + StringUtils.wrap(wizard.getMob().name().toLowerCase().replace("String_Node_Str","String_Node_Str")) + "String_Node_Str";
  if (spawnMob(player,npc)) {
    canSend=true;
  }
break;
case WIZARD_CHANGETIME:
msg+="String_Node_Str" + StringUtils.wrap(wizard.getTime()) + "String_Node_Str";
if (changeTime(player,npc)) {
canSend=true;
}
break;
case WIZARD_TOGGLESTORM:
msg+="String_Node_Str" + StringUtils.wrap(player.getWorld().getName()) + "String_Node_Str";
if (toggleStorm(player,npc)) {
canSend=true;
}
break;
default :
msg=ChatColor.RED + "String_Node_Str";
canSend=true;
break;
}
if (canSend) {
player.sendMessage(msg);
}
}
}
 else {
player.sendMessage(ChatColor.GRAY + "String_Node_Str");
}
}
 else if (EconomyHandler.useEconomy()) {
player.sendMessage(MessageUtils.getNoMoneyMessage(op,player));
}
}