When using a plugin to hide players from each other (like VanishNoPacket), particles from the hidden player are still watchable by other players.

To reproduce:
Make player A look at player B.
Make player B use creative mode and hide from player A.
Notice that player A can't see player B anymore.
With player B fly some blocks in the air, then fall down.
Notice that player A will see the generated particles on the landing site of player B.

Cause:
In WorldServer.sendParticles() the logic for canSee() ist reversed.