The bug

When you dismount a horse that is rearing, the horse keeps rearing until the client receives the next SPacketEntityMetadata packet.

How to reproduce

While riding a horse press the key assigned to JUMP for a very short moment and dismount the horse right after that
→ The horse keeps rearing for a while

The reason

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

I assume that the reason for this is that rearing is only done client side. The method net.minecraft.entity.passive.EntityHorse.onUpdate() contains a part that causes the horse to stop rearing. The following conditions are used. Keep in mind that they are all required and in Java if one condition fails all following conditions are not tested.

1. If controlling passenger is a player:
   1. True condition: Player is client player
   2. False condition: Code is run by server
2. Horse is rearing for more than 0 ticks
3. Add one to horse rearing time, new value has to be greater than 20

If all these conditions are met the rearing value is set to false. This means that as soon as the player dismounts the horse, the rearing time stops incrementing.

When the client receives a SPacketEntityMetadata packet, the rearing state updates, because the value if the horse is rearing or not is stored with other states in the data watcher as one value (single bits of the value representing the value of the states) and rearing is only client side.

Removing the condition that the controlling passenger has to be a player would probably fix this bug.