The bug

Items do not clip correctly to your arm when sneaking. A big example would be punching while sneaking.

How to reproduce

1. Put for example a sword in your mainhand
2. Give yourself mining fatigue to slow down your arm swing speed

   /effect give @s minecraft:mining_fatigue 30 4 true
   

3. Switch to the front view (press F5 twice)
4. Hit with the sword while sneaking