Minecraft 15w45a, Minecraft 15w51b
Using Boats and Ender Crystals, you are able to duplicate the content of your inventory as often as you want.
1. Make sure you are on a Multiplayer Server and in Gamemode 0
2. Get a setup like the following by stacking up on the side, placing a boat on a block above the Ender Crystal and remove the block afterwards.
3. Get into the boat (or stand on it) and wait a few seconds. You will get kicked for "Flying is not enabled on the server". If you get kicked from the server by (probably) every other reasons such as /kick, /ban, a timeout, normal logout, ..), it will work as well.
4. Relog a few times (the amount of relogs is the amount of duplicated inventories)
5. Get out of the boat and destroy the ender crystal
The server will think that you've died several times and drops the inventory. The player is however not listed anymore and cannot be targetted by using selectors in commands.
The player is not properly kicked from the server.
A Video showing the concept can be found here https://youtu.be/tZnQUtJwnqc