Program: 18942
Project: 18942
Buggy source:
@Override public void onPlayerInteract(PlayerInteractEvent event){
  if (event.hasBlock() && event.getClickedBlock().getType().equals(Material.STEP)) {
    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()) {
        if (showItem != null) {
          showItem.getExtra().onRightClick(player);
          return;
        }
 else {
          return;
        }
      }
      if (event.getPlayer().getLocation().getBlock().getFace(BlockFace.DOWN).getTypeId() == 0) {
        return;
      }
      if (event.hasBlock() && showItem == null && player.mayCreateHere(event.getClickedBlock())) {
        if (event.getItem() == null) {
          player.sendMessage(ShowcaseMain.tr("String_Node_Str"));
          event.setCancelled(true);
          return;
        }
        if (event.getClickedBlock().getType().equals(Material.STEP)) {
          event.setCancelled(true);
          if (ShowcaseMain.instance.providers.size() == 1 && ShowcaseMain.instance.providers.containsKey("String_Node_Str")) {
            Location loc=event.getClickedBlock().getLocation();
            Material mat=event.getItem().getType();
            short data=event.getItem().getDurability();
            ShowcaseMain.instance.showcasedItems.add(new ShowcaseItem(loc,mat,data,event.getPlayer().getName(),"String_Node_Str"));
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
          }
 else {
            ShowcaseCreationAssistant assistant=new ShowcaseCreationAssistant(event.getPlayer(),event.getItem(),event.getClickedBlock().getLocation());
            assistant.start();
          }
        }
      }
 else       if (showItem != null) {
        if (showItem.getPlayer().equals(event.getPlayer().getName()) || player.hasPermission("String_Node_Str",true)) {
          if (showItem.getExtra() == null) {
            showItem.remove();
            ShowcaseMain.instance.showcasedItems.remove(showItem);
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
            System.out.println("String_Node_Str");
            return;
          }
          if (showItem.getExtra().onDestroy(player)) {
            showItem.remove();
            ShowcaseMain.instance.showcasedItems.remove(showItem);
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
          }
        }
 else {
          event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str",showItem.getPlayer()));
        }
        event.setCancelled(true);
      }
    }
    if (event.getAction().equals(Action.LEFT_CLICK_BLOCK)) {
      if (showItem != null) {
        if (showItem.getExtra() != null)         showItem.getExtra().onClick(player);
      }
    }
  }
}

Fixed version (ground truth):
@Override public void onPlayerInteract(PlayerInteractEvent event){
  if (event.hasBlock() && event.getClickedBlock().getType().equals(Material.STEP)) {
    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()) {
        if (showItem != null) {
          showItem.getExtra().onRightClick(player);
          return;
        }
 else {
          return;
        }
      }
      if (event.getPlayer().getLocation().getBlock().getRelative(BlockFace.DOWN).getTypeId() == 0) {
        return;
      }
      if (event.hasBlock() && showItem == null && player.mayCreateHere(event.getClickedBlock())) {
        if (event.getClickedBlock().getType().equals(Material.STEP)) {
          event.setCancelled(true);
          if (ShowcaseMain.instance.providers.size() == 1 && ShowcaseMain.instance.providers.containsKey("String_Node_Str")) {
            Location loc=event.getClickedBlock().getLocation();
            Material mat=event.getItem().getType();
            short data=event.getItem().getDurability();
            ShowcaseMain.instance.showcasedItems.add(new ShowcaseItem(loc,mat,data,event.getPlayer().getName(),"String_Node_Str"));
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
          }
 else {
            ShowcaseCreationAssistant assistant=new ShowcaseCreationAssistant(event.getPlayer(),event.getItem(),event.getClickedBlock().getLocation());
            assistant.start();
          }
        }
      }
 else       if (showItem != null) {
        if (showItem.getPlayer().equals(event.getPlayer().getName()) || player.hasPermission("String_Node_Str",true)) {
          if (showItem.getExtra() == null) {
            showItem.remove();
            ShowcaseMain.instance.showcasedItems.remove(showItem);
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
            System.out.println("String_Node_Str");
            return;
          }
          if (showItem.getExtra().onDestroy(player)) {
            showItem.remove();
            ShowcaseMain.instance.showcasedItems.remove(showItem);
            event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str"));
          }
        }
 else {
          event.getPlayer().sendMessage(ShowcaseMain.tr("String_Node_Str",showItem.getPlayer()));
        }
        event.setCancelled(true);
      }
    }
    if (event.getAction().equals(Action.LEFT_CLICK_BLOCK)) {
      if (showItem != null) {
        if (showItem.getExtra() != null)         showItem.getExtra().onClick(player);
      }
    }
  }
}