If you are in a minecart in/directly under powdered snow that is not on a rail and press a movement key, the particles that come off of powdered snow when moving activates even though you aren't moving.
Steps to Reproduce: (from MC-220465 by @unknown)
Place a rail.
Place some powder snow above it.
Place a minecart onto the rail.
Destroy the rail.
Get inside the minecart.
Move forward.
→ ❌ Notice how particles are produced, despite yourself not moving at all.
Link to video of the bug:
Can confirm for 21w11a.