DataWatcher / EntityMetadata ID is based on the class load order ! Can cause many issues !

Example:
All dropped items will display as stone. DataWatcher ID bug.
https://www.youtube.com/watch?v=788153dV8Hk

Reproduce bug:
You just have to throw a splash potion before viewing any ground item (since your Minecraft was started) !

Then, go on a Minecraft Server (Vanilla, Spigot 1.9 or servers with 1.8/1.9 support) and drop an item on ground or throw a potion.
The item will be stone and the potion will display as water without real potion color.

In logs you will see:

• "Item entity XXX has no item?!"

• "ThrownPotion entity XXX has no item?!"

Cause:
This bug append when EntityPotion class was loaded before EntityItem class (so, when you see throwed potion before item on ground).
That's because DataWatcher/EntityMetadata ID was based on class load order in some cases.

Code is from Spigot 1.9, but it's the same on 1.9 client :

EntityItem.java

private static final DataWatcherObject<Optional<ItemStack>> c = DataWatcher.a(EntityItem.class, DataWatcherRegistry.f);

EntityPotion.java

private static final DataWatcherObject<Optional<ItemStack>> d = DataWatcher.a(EntityItem.class, DataWatcherRegistry.f);

This two codes call 'DataWatcher.a(EntityItem.class, DataWatcherRegistry.f);':

public static <T> DataWatcherObject<T> a(Class<? extends Entity> oclass, DataWatcherSerializer<T> datawatcherserializer)
  {
    int i;
    int i;
    if (a.containsKey(oclass))
    {
      i = ((Integer)a.get(oclass)).intValue() + 1;
    }
    else
    {
      int j = 0;
      Class oclass1 = oclass;
      while (oclass1 != Entity.class)
      {
        oclass1 = oclass1.getSuperclass();
        if (a.containsKey(oclass1))
        {
          j = ((Integer)a.get(oclass1)).intValue() + 1;
          break;
        }
      }
      i = j;
    }
    if (i > 254) {
      throw new IllegalArgumentException("Data value id is too big with " + i + "! (Max is " + 254 + ")");
    }
    a.put(oclass, Integer.valueOf(i));
    return datawatcherserializer.a(i);
  }

As you can see, variable i (the datawatcher index) is based on previous oclass value + 1.

So, if EntityItem load first, the id will be 5 and then when EntityPotion load the id will be 6.
But when EntityPotion load first the inverse occurs !

And when the inverse occurs the datawatcher ID was inverse and can't be synchronized with Minecraft servers.