The bug

When any kind of arrow is shot using a crossbow that is enchanted with Piercing at an entity, and the mob passes through fewer entities than the Piercing enchantment allows, it will stick in the ground and can be picked back up again.

This can be abused by using tipped arrows and shooting yourself in order to obtain infinite potion effects.

To reproduce

1. Get a crossbow
2. Enchant it with Piercing
3. Switch into Survival mode (if you aren't already)
4. Load the crossbow with an arrow
5. Shoot the arrow at an entity
   → The arrow will be able to be picked up after it hit the entity and can be used again

Original description

A bug with the Piercing enchantment.
when you fire a tipped arrow in the air with a piercing crossbow and hit yourself with the arrow, it will give you the potion effect and pierce you, hitting the ground allowing you to then pick up the arrow and use it again.