Every now and then, some things, such as rain, snow, fire, crimson spores, warped spores, etc., become jittery. This even applies to the player's hand when switching items.

How to reproduce:

Step 1: Go into your world.

Step 2: Wait a few seconds or minutes.

Step 3: Watch everything become jittery.

How to (temporarily) fix:

Go into your inventory and then exit back out.