The bug

Using rotten flesh to breed wolves, upon a successful breeding, will cause the player who caused the breeding to have 25 seconds of hunger.

This is new to me for 1.9.

How to reproduce

1. Make sure you are in survival mode and don't have full hunger bar
2. Hurt a tamed wolf
3. Right click the wolf twice (after the second click hearts should appear)
4. Wait
   → Hunger is applied

Code analysis

The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.

It looks like there are multiple problems which cause this bug. One is that the item in use and the item in the selected slot are not the same item object (see also MC-86252 and this comment). This causes the method net.minecraft.entity.EntityLivingBase.func_184608_ct() to reset the item in use. It should however as well send the action to the server net.minecraft.network.play.client.CPacketPlayerDigging.Action.RELEASE_USE_ITEM. The second problem is that the item in use and the item in the hotbar are desynced (client side), this can be seen when you right click the wolf, the first right click does nothing but consumes one rotten flesh, only the second one creates the hearts. This causes then later the method net.minecraft.entity.EntityLivingBase.func_184608_ct() to reset the item in use, because the item in use still has still the old count whereas the selected item has already the new count.