Steps to Reproduce:
1. run /worldborder set 100
2. place one block adjacent to the world border and another inward
3. place a piston such that it will push the second block into the first
4. activate the piston
5. The two blocks will Z fight
If the world border is moved after this, the ghost block will move to its actual position as a moving_piston.
Related to MC-158821
This ticket should most likely be resolved as a duplicate of MC-82010.