When I cancel an inventory click, players can still shift-click a barrier in the inventory and have it added to their inventory. This doesn't seem to happen with any other item in the inventory (that I know of) and no it's not me forgetting to cancel when the player clicks a barrier, I cancel the event after I check if it is my custom inventory. Another thing I noticed is I don't think players actually have the item. Once the barrier is added to my inventory I can't actually place it and when I leave the server and rejoin the item is gone.