First THANKS to lishid for his MARVELOUS plugin OpenInv : OpenInventory.
Without this plugin neither the idea, concept or coding this feature would have been possible. Then THANKS for his hard work.
With the command
/oi) (and the right node, see Player Commands) you can open every player's inventory, and play with it.
The Inventory will be open as a chest where you'll be able to interact with it.
BE CAREFUL the 5 lasts slots are not used, if you put an item in it, they will be deleted ! (see the images)
You want to check if a user is not doing something wrong after a new inscription. You can then follow him with invisibility and check everything he take in chest with OpenInv. You can also do some kind of funny things (see above)
From OpenInv plugin : OpenInv bukkit plugin