The bug

When you're swimming and you suddenly stop to charge a trident to shoot, you will not only see MC-131286 in action but you will also be able to appreciate that the arm that is holding the trident will reduce its size and will also show its "inner" part to the exterior. Please note that this only occurs while having your left hand selected as the main hand.

How to reproduce

1. Open/create a world - preferably using the "Water world" preset as it will make it easier to reproduce
2. Change your main hand to left
3. Hold a trident on your main hand and equip a shield on the other
4. Start swimming and then charge the trident (stop pressing "W" and "CTRL")
   →  You will see how the trident arm is way smaller and showing the "inner" part of your skin, the one that would normally face the chest