After I was killed by my friend, they crouched and I noticed something very odd. The pose/animation of my hand is now in sync with their posing. When they crouch, my hand moves as if I were crouching. When they pull back on a bow, my hand reaches down as if to do the same. Nothing is selected on my hotbar.

Not entirely sure how to reproduce but this doesn't seem to be a simple glitch from sync issues.

Additionally, simply logging out and logging back into the same server does not fix the issue, which is very odd. Restarting the entire client does not fix the bug either. It seems my server has a permanent connection between our IDs now.

Since then, I've now killed her as well and we are both bound to each other (how lovely!). If one of us has an empty hand, we'll see the viewmodel angle of the other player. Nothing appears in our hands, but the hand moves to compensate.

Restarted the server as well, and that didn't fix it either. As long as she is in my render distance and her name renders overhead in my client, my hand will be tied to her. If we're in different parts of the world and can't see each other then the hand acts as normal.

From MC-46797:
Only tested in LAN multiplayer

Problem

When the other player(s) equip an item or crouch your character's hands move. This only occurs when you can see the other player(s) in your screen (field of vision). A direct line of sight is not required as long as you're pointing towards another player.

Attachments

2014-02-02_11.21.35 = Normal; no-one is holding anything or crouching (default)

2014-02-02_11.21.42 = Other player is holding an Item (affecting your character's hand position)

2014-02-02_11.21.50 = Other player is crouching (affecting your character's hand position)

2014-02-02_11.22.36 = Other player is crouching (affecting your character's hand position at a distance)