If you shoot an item frame with a crossbow that has a Piercing level of at least one, the arrow shot by that crossbow can pierce through the wall the item frame was placed on.
- Build a wall that is one block thick
- Place an item frame on that wall
- Shoot the item frame with an unenchanted crossbow
→ The arrow doesn't pierce through the wall / disappears
- Replace the item frame with a new one
- Shoot the item frame with a crossbow that has at least Piercing I
→ The arrow can pierce through the wall