Crossbow arrows will "bounce" off of mobs back towards the player instead of damaging (or piercing) them.

How to Reproduce

It's a bit inconsistent, but there are a few ways to make it occur more frequently.
1. summon a bunch of mobs
2. Use a crossbow with Quickdraw 5 and multishot

3. Start spamming

While I have no way of confirming this, it appears that it happens when the arrow "grazes" the side of the mob hitbox.