If an item is held in hand, and a player swaps to any object that can be placed, an instance of the block will appear, before reverting, and the hotbar will automatically get put back on the slot prior to the hotkey. For instance, holding an item like a pickaxe in slot one, with a stack of blocks in slot two. If you hotkey to two, while right-clicking to place a block, the block will not place, and you will remain on hotbar slot one.