This bug occurs while in-game on Summoner's Rift, as far as my research goes doesn't affect any particular champion. Randomly, whenever you would drag one of your items from a slot to another, the game will instead pick up the item's image instead of the item itself. The item stays in the same slot it was in, but the image can be put anywhere on the screen and will stay there.
Only remedy I found was picking up another item (let's call it item Z) and putting it in the bugged item's (let's call it item Y) slot (which isn't enough as item Y is now bugged just like item Z, but now item Z is no longer bugged). Now you swap the 2 items again and hopefully it's fixed. I've tried reproducing the bug multiple times, but it seemed to happen randomly, and testing on Practice Tool as well as Custom Games yielded the same result.
So far this bug has happened at least once-twice per game I've played, so it's not really scarce, and it denies you the ability to relocate your items as the image doesn't count as item relocation. I haven't been able to take pictures of it, but whenever I do, I will immediately post it on this thread for easy understanding.
EDIT: From what I recall and the current replies to the thread, it seems the bug happens when you have a support item in your inventory. If you do, the bug mentioned above starts happening.