A potion I made with the new /give dataTags that crashes the game. If you hover over the item in your inventory (or if you select the item in your hotbar) Minecraft will crash. If you select it in your hotbar, every time you reconnect to the server or load the world, you will immediately crash. This potion can be used as a ban/kick potion and allows you to ban/kick players with Command Blocks. Server OPs can crash a player's game whenever they want with this potion. This potion makes singleplayer worlds unplayable if selected in the hotbar because your game crashes every time you load the world.
Edit: I have noticed that the crash reports are different when you select the item in the hotbar and when you hover over the item in your inventory. Added two crash reports.

Type this into a Command Block and run the command to recreate (please create a new world and do this, don't take a chance of ruining one of your worlds):

/give @p 373 64 8236 {CustomPotionEffects:[{Id:8236,Amplifier:10,Duration:40},{Id:8,Amplifier:20,Duration:5000}],display:{Name:"Potion of Crashing",Lore:["Crashes your game","Insert lore here"]}}