When you try to set a random entity on a player's head by using setPassenger, the passenger doesn't get shown and stays at it's place.

1.8.x
player.setPassenger(entity)

https://www.spigotmc.org/attachments/2016-03-11_18-46-09-png.111603/

1.9.0

https://www.spigotmc.org/threads/spigot-1-9-player-setpassenger-help.130324/#post-1386915

Solution and more information about the error:

https://www.spigotmc.org/threads/spigot-1-9-player-setpassenger-help.130324/#post-1386915