The sparkle coming from the feet of the new headless horseman mount.
On the ground or with static flight it has sparkles coming from it’s feet when you move but when skyriding these disapear.
I would guess its because of the people that seize (or just get HA or really uncomfortable) from the screen flicker cause by such effects during skyriding. There have been some threads on the topic.
Same thing happens with the priest class hall mount. I have to assume it’s something to do with the skyriding speed effects themselves.
But it’s just as well. Crap flying off your character while doing that is incredibly annoying. Just play a priest with default shadowform, and all that purple smoke flying in your face if the camera is behind your character.
I sadly would not know this is a thing. 
should be an interface option then.