I found this bug while playing on 2 accounts, Here is how it
works.

If you hold a item/block with your mouse cursor in the inventory
(In Survival, Adventure)
And then if another player/account or a commandblock
does /clear to you, then you will see the block is still in your
mouse cursor, but its a sort of "Ghost Block" becuase it isnt
effected with /clear anymore and if you try to use/place the item/block
then it wond work, because if its a block then it deseperas after placing.

How to do this:

Get into Survival or Adventure and get a block or item, hold it
in the mouse cursor.

Get another account and make him do /clear (Username of player holding the block)

Done. Not try to place the block....