Program: 178715
Project: 178715
Buggy source:
public void onBlockPistonExtend(BlockPistonExtendEvent event){
  if (event.isCancelled())   return;
  Region region=plugin.regionManager.getRegion(new V10Location(event.getBlock()));
  for (  Block b : event.getBlocks()) {
    if (blockedPistonBlocks.contains(b)) {
      event.setCancelled(true);
      return;
    }
    Portal portal=plugin.portalManager.insideBlocks.get(b.getRelative(event.getDirection()).getLocation());
    if (portal != null && region.getBoolean(RegionSetting.ENABLE_PISTON_BLOCK_TELEPORT)) {
      Portal destP=portal.getDestination();
      final Block destB=destP.teleport.getHandle().getBlock();
      if (!portal.open || !destP.open) {
        event.setCancelled(true);
        return;
      }
      if (destB.isLiquid() || destB.getType() == Material.AIR) {
        final Block endBlock=b.getRelative(event.getDirection());
        blockedPistonBlocks.add(endBlock);
        plugin.getServer().getScheduler().scheduleSyncDelayedTask(plugin,new Runnable(){
          public void run(){
            destB.setType(endBlock.getType());
            destB.setData(endBlock.getData(),false);
            endBlock.setType(Material.AIR);
            blockedPistonBlocks.remove(endBlock);
          }
        }
,2L);
      }
 else {
        event.setCancelled(true);
      }
    }
 else     if (plugin.portalManager.borderBlocks.containsKey(b.getLocation()) || plugin.grillManager.borderBlocks.containsKey(b.getLocation()) || plugin.grillManager.insideBlocks.containsKey(b.getLocation())) {
      event.setCancelled(true);
    }
  }
}

Fixed version (ground truth):
public void onBlockPistonExtend(BlockPistonExtendEvent event){
  if (event.isCancelled())   return;
  Region region=plugin.regionManager.getRegion(new V10Location(event.getBlock()));
  for (  Block b : event.getBlocks()) {
    if (blockedPistonBlocks.contains(b)) {
      event.setCancelled(true);
      return;
    }
    Portal portal=plugin.portalManager.insideBlocks.get(new V10Location(b.getRelative(event.getDirection()).getLocation()));
    if (portal != null && region.getBoolean(RegionSetting.ENABLE_PISTON_BLOCK_TELEPORT)) {
      Portal destP=portal.getDestination();
      final Block destB=destP.teleport.getHandle().getBlock();
      if (!portal.open || !destP.open) {
        event.setCancelled(true);
        return;
      }
      if (destB.isLiquid() || destB.getType() == Material.AIR) {
        final Block endBlock=b.getRelative(event.getDirection());
        blockedPistonBlocks.add(endBlock);
        plugin.getServer().getScheduler().scheduleSyncDelayedTask(plugin,new Runnable(){
          public void run(){
            destB.setType(endBlock.getType());
            destB.setData(endBlock.getData(),false);
            endBlock.setType(Material.AIR);
            blockedPistonBlocks.remove(endBlock);
          }
        }
,2L);
      }
 else       event.setCancelled(true);
    }
 else {
      V10Location loc=new V10Location(b.getLocation());
      if (plugin.portalManager.borderBlocks.containsKey(loc) || plugin.grillManager.borderBlocks.containsKey(loc) || plugin.grillManager.insideBlocks.containsKey(loc))       event.setCancelled(true);
    }
  }
}