A player with a backpack, picks up another backpack and places it inside of their backpack (or buys one from a trader in certain game modes).
Then, the player sets their backpack on the ground (with the second backpack still inside).
Then, they double click the exterior backpack which shows them the contents.
They right click the interior backpack which attempts, but fails, to place it onto their backpack slot.
They exit their inventory completely and use the "grab" scroll option to grab the exterior backpack.
Their game will immediately freeze up and the server will crash.