This bug may not be a problem for most players, but it probably will be for mapmakers. The problem caused by this bug is that when you use /entitydata for the "Riding" tag, the output will say that it has been changed, but the mob won't change. One example that this bug is a problem for, is things like boss fights. For example: "/entitydata @e[type=ArmorStand,name=Boss] {Riding:{id:Zombie}}". In this case, you would need to kill the armor stand named "Boss" to kill the zombie, but nothing happens.