Items that are located in a place where a shelf is going to be placed start moving until someone breaks the shelf or the item gets collected

To reproduce
1. Put a line of three solid blocks behind the place where you want to put the shelf.
2. Put two shelves, one on the block on the left and another one on the block on the right
2. Put an item on the floor where there isn't a shelf, being located as near as possible from the blocks
3. Put a shelf on the centre
The item placed will start moving undefinitely until one of the conditions of the description is met