Hidden inventory items

I only tried this on ARAM. If you open up the shop, hover the shop window over your top row of items, and drag an item from the bottom row onto an item in the top row, instead of swapping the two items, the item you dragged will vanish. It'll still exist in your inventory, as you can still hover over it and see its description, it just doesn't show up.

You make make it re-appear by clicking on the missing item.

In my picture, the empty inventory slot isn't actually empty. I forgot to hover over the item when I took the screenshot :^)