Found:
An arrow (stuck in a block) will shake when world loads.

Expected:
That the arrow would be still.

Steps:
0. Set your render distance to max. Lower, and it can be unnoticeable.
1. Shoot an arrow into a block nearby.
2. Leave the world.
3. Open the world.
4. The moment the arrow loads and you can see it, it shakes.
5. This is separate from the arrow slightly jumping after a second, which seems to be MC-85274 – "When an arrow lands on the ground, it hangs to the side for a second". If your render distance is too low, it's possible you'll only see the one slight jump. Raise render distance to max, and you ought to see the shake before the the jump.

May be related to: MC-3352 – "Arrows fired into a repeater / comparator / jukebox / command block that is updating will play the arrow hitting a block noise."