The damage from a Piercing arrow is randomized every time it passes through a mob, instead of remaining the same after being shot from the crossbow.

To reproduce

• Obtain a Piercing IV crossbow
• Summon 5 or less 20-health mobs (e.g. creepers) lined up with {NoAI:1b}
• Shoot the crossbow so that the arrow hits all of the mobs
• Repeat several times

As shown in this video, some mobs will die after taking two hits, while others take three; the mobs that die change every time this is repeated. The remaining health of the mobs that survive two hits also changes every time, usually between 1-4 HP. This could make advancements such as "Two Birds, One Arrow" and "Arbalistic" trickier to get, since the mobs could take a different number of hits to kill despite starting out with the same health and being damaged by the same arrow.

Code analysis

Crossbow arrows shot by players always do critical damage; the random critical damage bonus is calculated on every hit instead of once when the arrow is spawned.