Program: 10151
Project: 10151
Buggy source:
public void handle(Packet packet){
  if (packet instanceof PacketPlayOutChat) {
    if (!packetHandler.sendPacket(player.getBukkitEntity(),new PacketOutChat_v1_8_R3((PacketPlayOutChat)packet))) {
      oldManager.handle(packet);
    }
  }
 else   if (packet instanceof PacketPlayOutNamedEntitySpawn || packet instanceof PacketPlayOutSpawnEntity || packet instanceof PacketPlayOutSpawnEntityLiving|| packet instanceof PacketPlayOutSpawnEntityPainting|| packet instanceof PacketPlayOutSpawnEntityExperienceOrb) {
    PacketOutSpawnEntity spawnEntity=new PacketOutSpawnEntity_v1_8_R3(player,packet);
    UUID uuid=spawnEntity.getEntityUuid();
    if (!NMSHandler.getInstance().getEntityHelper().isHidden(player.getBukkitEntity(),uuid)) {
      Entity entity=((WorldServer)player.getWorld()).getEntity(uuid);
      if (entity instanceof EntityFakePlayer_v1_8_R3) {
        final EntityFakePlayer_v1_8_R3 fakePlayer=(EntityFakePlayer_v1_8_R3)entity;
        handle(new PacketPlayOutPlayerInfo(EnumPlayerInfoAction.ADD_PLAYER,fakePlayer));
        Bukkit.getScheduler().runTaskLater(NMSHandler.getJavaPlugin(),new Runnable(){
          @Override public void run(){
            handle(new PacketPlayOutPlayerInfo(EnumPlayerInfoAction.REMOVE_PLAYER,fakePlayer));
          }
        }
,5);
      }
      oldManager.handle(packet);
    }
  }
 else   if (packet instanceof PacketPlayOutEntityMetadata) {
    if (!packetHandler.sendPacket(player.getBukkitEntity(),new PacketOutEntityMetadata_v1_8_R3((PacketPlayOutEntityMetadata)packet))) {
      oldManager.handle(packet);
    }
  }
 else {
    oldManager.handle(packet);
  }
}

Fixed version (ground truth):
public void handle(Packet packet){
  if (packet instanceof PacketPlayOutChat) {
    if (!packetHandler.sendPacket(player.getBukkitEntity(),new PacketOutChat_v1_8_R3((PacketPlayOutChat)packet))) {
      oldManager.handle(packet);
    }
  }
 else   if (packet instanceof PacketPlayOutNamedEntitySpawn || packet instanceof PacketPlayOutSpawnEntity || packet instanceof PacketPlayOutSpawnEntityLiving|| packet instanceof PacketPlayOutSpawnEntityPainting|| packet instanceof PacketPlayOutSpawnEntityExperienceOrb) {
    PacketOutSpawnEntity spawnEntity=new PacketOutSpawnEntity_v1_8_R3(player,packet);
    UUID uuid=spawnEntity.getEntityUuid();
    if (!NMSHandler.getInstance().getEntityHelper().isHidden(player.getBukkitEntity(),uuid)) {
      Entity entity=((WorldServer)player.getWorld()).getEntity(uuid);
      if (entity != null) {
        if (entity instanceof EntityFakePlayer_v1_8_R3) {
          final EntityFakePlayer_v1_8_R3 fakePlayer=(EntityFakePlayer_v1_8_R3)entity;
          handle(new PacketPlayOutPlayerInfo(EnumPlayerInfoAction.ADD_PLAYER,fakePlayer));
          Bukkit.getScheduler().runTaskLater(NMSHandler.getJavaPlugin(),new Runnable(){
            @Override public void run(){
              handle(new PacketPlayOutPlayerInfo(EnumPlayerInfoAction.REMOVE_PLAYER,fakePlayer));
            }
          }
,5);
        }
      }
      oldManager.handle(packet);
    }
  }
 else   if (packet instanceof PacketPlayOutEntityMetadata) {
    if (!packetHandler.sendPacket(player.getBukkitEntity(),new PacketOutEntityMetadata_v1_8_R3((PacketPlayOutEntityMetadata)packet))) {
      oldManager.handle(packet);
    }
  }
 else {
    oldManager.handle(packet);
  }
}