Everyone had probably heard of this by now. Within the last patch or two there’s a dupe you can do with the pistons. It works on both single player and multiplayer and lets you dupe any item which you can place down as a block. The most common usage for this has been with diamond and TnT (now you can break it).
Especially in PvP and Faction minecraft servers – this has been done to death so there’s a chance your server has already disabled it but it doesn’t hurt to try and see.