Minecraft 14w05b, Minecraft 14w11b
In a non-spectator mode, I used a timed potion on myself (thrown). I then switched to spectator mode and cleared all effects with -
/effects Bmalchert clear
pressed f5, noticed my spectator head was still emitting particles (though not as much particles as when in another gamemode), and screamed.
Changed to another non-spectator gamemode and particles dissipated shortly after.
When this happens other players cannot to see the potion particles of the spectator, only the spectator can.