Program: 18962
Project: 18962
Buggy source:
@Override public void onPlayerInteract(PlayerInteractEvent event){
  if (event.hasBlock()) {
    ShowcaseItem showItem=ShowcaseMain.instance.getItemByBlock(event.getClickedBlock());
    ShowcasePlayer player=ShowcasePlayer.getPlayer(event.getPlayer());
    if (event.getAction().equals(Action.RIGHT_CLICK_BLOCK)) {
      if (!event.getPlayer().isSneaking()) {
        return;
      }
      if (event.getItem() == null) {
        player.sendMessage(ChatColor.RED + "String_Node_Str");
        event.setCancelled(true);
        return;
      }
      if (event.hasBlock() && event.hasItem() && showItem == null) {
        if (event.getClickedBlock().getType().equals(Material.GLASS)) {
          event.setCancelled(true);
          if (!isSafePlace(event.getClickedBlock())) {
            player.sendMessage(ChatColor.RED + "String_Node_Str");
            return;
          }
          printTypeMenu(event.getPlayer());
          player.setDialogState(1);
          player.setRequestedItem(event.getItem().clone());
          player.setRequestedBlock(event.getClickedBlock());
        }
      }
 else       if (showItem != null) {
        if (showItem.getPlayer().equals(event.getPlayer().getName())) {
          showItem.remove();
          ShowcaseMain.instance.showcasedItems.remove(showItem);
          event.getPlayer().sendMessage(ChatColor.RED + "String_Node_Str");
        }
 else {
          event.getPlayer().sendMessage(ChatColor.RED + "String_Node_Str" + showItem.getPlayer()+ "String_Node_Str");
        }
        event.setCancelled(true);
      }
    }
 else     if (event.getAction().equals(Action.LEFT_CLICK_BLOCK)) {
      if (showItem != null && showItem.getType().toString().contains("String_Node_Str")) {
        if (!player.hasReadPrice() || !player.standsOnReadPosition()) {
          String print=ChatColor.YELLOW + "String_Node_Str" + ChatColor.WHITE+ showItem.getPricePerItem()+ "String_Node_Str";
          player.setHasReadPrice(true);
          player.setReadPriceLocation(player.getPlayer().getLocation());
          if (showItem.getType().equals(ShowcaseType.FINITE_SHOP)) {
            print+=ChatColor.YELLOW + "String_Node_Str" + ChatColor.WHITE+ showItem.getItemAmount();
            print+=ChatColor.YELLOW + "String_Node_Str";
          }
 else           if (showItem.getType().equals(ShowcaseType.INFINITE_SHOP)) {
            print+=ChatColor.YELLOW + "String_Node_Str";
          }
          player.sendMessage(print);
        }
 else {
          if (player.standsOnReadPosition()) {
            ItemStack stack=showItem.getItem().getItemStack().clone();
            stack.setAmount(1);
            if (player.withdraw(showItem.getPricePerItem())) {
switch (showItem.getType()) {
case INFINITE_SHOP:
                if (player.hasPermission("String_Node_Str",false)) {
                  player.getPlayer().getInventory().addItem(stack);
                  player.sendMessage("String_Node_Str");
                }
 else {
                  player.sendMessage("String_Node_Str");
                }
              break;
case FINITE_SHOP:
            if (player.hasPermission("String_Node_Str",false)) {
              player.getPlayer().getInventory().addItem(stack);
              showItem.setItemAmount(showItem.getItemAmount() - 1);
              ShowcasePlayer owner=ShowcasePlayer.getPlayer(showItem.getPlayer());
              owner.giveMoney(showItem.getPricePerItem());
              player.sendMessage("String_Node_Str" + showItem.getItemAmount());
            }
 else {
              player.sendMessage("String_Node_Str");
            }
          break;
      }
    }
 else {
      player.sendMessage("String_Node_Str");
    }
  }
 else {
    player.setHasReadPrice(false);
  }
}
}
}
}
}

Fixed version (ground truth):
@Override public void onPlayerInteract(PlayerInteractEvent event){
  if (event.hasBlock()) {
    ShowcaseItem showItem=ShowcaseMain.instance.getItemByBlock(event.getClickedBlock());
    ShowcasePlayer player=ShowcasePlayer.getPlayer(event.getPlayer());
    if (event.getAction().equals(Action.RIGHT_CLICK_BLOCK)) {
      if (!event.getPlayer().isSneaking()) {
        return;
      }
      if (event.hasBlock() && showItem == null) {
        if (event.getItem() == null) {
          player.sendMessage(ChatColor.RED + "String_Node_Str");
          event.setCancelled(true);
          return;
        }
        if (event.getClickedBlock().getType().equals(Material.GLASS)) {
          event.setCancelled(true);
          if (!isSafePlace(event.getClickedBlock())) {
            player.sendMessage(ChatColor.RED + "String_Node_Str");
            return;
          }
          if (player.hasPermission("String_Node_Str",false) && !player.hasPermission("String_Node_Str",true) && !player.hasPermission("String_Node_Str",false)) {
            Location loc=event.getClickedBlock().getLocation();
            Material mat=event.getItem().getType();
            short data=event.getItem().getDurability();
            addShowcase(loc,mat,data,player.getPlayer(),ShowcaseType.BASIC,1,0);
            player.sendMessage(ChatColor.GREEN + "String_Node_Str" + mat+ "String_Node_Str");
            player.resetDialog();
          }
 else {
            printTypeMenu(event.getPlayer());
            player.setDialogState(1);
            player.setRequestedItem(event.getItem().clone());
            player.setRequestedBlock(event.getClickedBlock());
          }
        }
      }
 else       if (showItem != null) {
        if (showItem.getPlayer().equals(event.getPlayer().getName())) {
          showItem.giveItemsBack();
          showItem.remove();
          ShowcaseMain.instance.showcasedItems.remove(showItem);
          event.getPlayer().sendMessage(ChatColor.RED + "String_Node_Str");
        }
 else {
          event.getPlayer().sendMessage(ChatColor.RED + "String_Node_Str" + showItem.getPlayer()+ "String_Node_Str");
        }
        event.setCancelled(true);
      }
    }
 else     if (event.getAction().equals(Action.LEFT_CLICK_BLOCK)) {
      if (showItem != null && showItem.getType().toString().contains("String_Node_Str")) {
        if (!player.hasReadPrice() || !player.standsOnReadPosition()) {
          String print=ChatColor.YELLOW + "String_Node_Str" + ChatColor.WHITE+ showItem.getPricePerItem()+ "String_Node_Str";
          player.setHasReadPrice(true);
          player.setReadPriceLocation(player.getPlayer().getLocation());
          if (showItem.getType().equals(ShowcaseType.FINITE_SHOP)) {
            print+=ChatColor.YELLOW + "String_Node_Str" + ChatColor.WHITE+ showItem.getItemAmount();
            print+=ChatColor.YELLOW + "String_Node_Str";
          }
 else           if (showItem.getType().equals(ShowcaseType.INFINITE_SHOP)) {
            print+=ChatColor.YELLOW + "String_Node_Str";
          }
          player.sendMessage(print);
        }
 else {
          if (player.standsOnReadPosition()) {
            ItemStack stack=showItem.getItem().getItemStack().clone();
            stack.setAmount(1);
            if (player.withdraw(showItem.getPricePerItem())) {
switch (showItem.getType()) {
case INFINITE_SHOP:
                if (player.hasPermission("String_Node_Str",false)) {
                  player.getPlayer().getInventory().addItem(stack);
                  player.sendMessage("String_Node_Str");
                }
 else {
                  player.sendMessage("String_Node_Str");
                }
              break;
case FINITE_SHOP:
            if (player.hasPermission("String_Node_Str",false)) {
              player.getPlayer().getInventory().addItem(stack);
              showItem.setItemAmount(showItem.getItemAmount() - 1);
              ShowcasePlayer owner=ShowcasePlayer.getPlayer(showItem.getPlayer());
              owner.giveMoney(showItem.getPricePerItem());
              player.sendMessage("String_Node_Str" + showItem.getItemAmount());
            }
 else {
              player.sendMessage("String_Node_Str");
            }
          break;
      }
    }
 else {
      player.sendMessage("String_Node_Str");
    }
  }
 else {
    player.setHasReadPrice(false);
  }
}
}
}
}
}