Program: 11676
Project: 11676
Buggy source:
@Override public final void setPosition(Position position){
  Position old=this.position;
  this.position=position;
  TridentWorld fromWorld=(TridentWorld)old.getWorld();
  TridentWorld destWorld=(TridentWorld)position.getWorld();
  if (!destWorld.equals(fromWorld)) {
    if (this instanceof Player) {
      fromWorld.getOccupants().remove(this);
      destWorld.getOccupants().add((TridentPlayer)this);
    }
 else {
      fromWorld.getEntitySet().remove(this);
      destWorld.getEntitySet().add(this);
    }
  }
  int destCX=position.getChunkX();
  int destCZ=position.getChunkZ();
  TridentChunk destChunk=destWorld.getChunkAt(destCX,destCZ,true);
  int fromCX=old.getChunkX();
  int fromCZ=old.getChunkZ();
  if (fromCX != destCX || fromCZ != destCZ) {
    TridentChunk fromChunk=fromWorld.getChunkAt(fromCX,fromCZ,false);
    List<Entity> destroy=Collections.singletonList(this);
    if (this instanceof Player) {
      if (fromChunk == null) {
        throw new RuntimeException("String_Node_Str");
      }
      TridentPlayer player=(TridentPlayer)this;
      fromChunk.getOccupants().remove(player);
      destChunk.getOccupants().add(player);
      Stream.concat(fromChunk.getHolders().stream(),destChunk.getHolders().stream()).distinct().forEach(p -> {
        if (!fromChunk.getHolders().contains(p)) {
          p.net().sendPacket(new PlayOutSpawnPlayer(player));
        }
        if (!destChunk.getHolders().contains(p)) {
          p.net().sendPacket(new PlayOutDestroyEntities(destroy));
        }
      }
);
      player.updateChunks();
    }
 else {
      if (fromChunk != null) {
        fromChunk.getEntitySet().remove(this);
      }
      destChunk.getEntitySet().add(this);
      Stream.concat(fromChunk.getHolders().stream(),destChunk.getHolders().stream()).distinct().forEach(p -> {
        if (!fromChunk.getHolders().contains(p)) {
          p.net().sendPacket(this.getSpawnPacket());
        }
        if (!destChunk.getHolders().contains(p)) {
          p.net().sendPacket(new PlayOutDestroyEntities(destroy));
        }
      }
);
    }
  }
  Position delta=position.subtract(old);
  if (delta.getX() != 0 || delta.getY() != 0 || delta.getZ() != 0) {
    if (Double.compare(old.getYaw(),position.getYaw()) == 0 && Double.compare(old.getPitch(),position.getPitch()) == 0) {
      PlayOutEntityRelativeMove packet=new PlayOutEntityRelativeMove(this,delta);
      RecipientSelector.whoCanSee(destChunk,packet,this);
    }
 else {
      PlayOutEntityLookAndRelativeMove lookAndRelativeMove=new PlayOutEntityLookAndRelativeMove(this,delta);
      RecipientSelector.whoCanSee(destChunk,lookAndRelativeMove,this);
      PlayOutEntityHeadLook look=new PlayOutEntityHeadLook(this);
      RecipientSelector.whoCanSee(destChunk,look,this);
    }
  }
 else   if (old.getYaw() != position.getYaw() || old.getPitch() != position.getPitch()) {
    PlayOutEntityLookAndRelativeMove lookAndRelativeMove=new PlayOutEntityLookAndRelativeMove(this,delta);
    RecipientSelector.whoCanSee(destChunk,lookAndRelativeMove,this);
    PlayOutEntityHeadLook look=new PlayOutEntityHeadLook(this);
    RecipientSelector.whoCanSee(destChunk,look,this);
  }
}

Fixed version (ground truth):
@Override public final void setPosition(Position position){
  Position old=this.position;
  this.position=position;
  TridentWorld fromWorld=(TridentWorld)old.getWorld();
  TridentWorld destWorld=(TridentWorld)position.getWorld();
  if (!destWorld.equals(fromWorld)) {
    if (this instanceof Player) {
      fromWorld.getOccupants().remove(this);
      destWorld.getOccupants().add((TridentPlayer)this);
    }
 else {
      fromWorld.getEntitySet().remove(this);
      destWorld.getEntitySet().add(this);
    }
  }
  int destCX=position.getChunkX();
  int destCZ=position.getChunkZ();
  TridentChunk destChunk=destWorld.getChunkAt(destCX,destCZ);
  int fromCX=old.getChunkX();
  int fromCZ=old.getChunkZ();
  if (fromCX != destCX || fromCZ != destCZ) {
    TridentChunk fromChunk=fromWorld.getChunkAt(fromCX,fromCZ,false);
    List<Entity> destroy=Collections.singletonList(this);
    PacketOut spawnThis=this.getSpawnPacket();
    if (this instanceof Player) {
      TridentPlayer player=(TridentPlayer)this;
      if (fromChunk != null) {
        fromChunk.getOccupants().remove(player);
      }
      destChunk.getOccupants().add(player);
      Stream.concat(fromChunk == null ? Stream.empty() : fromChunk.getHolders().stream(),destChunk.getHolders().stream()).distinct().forEach(p -> {
        if (fromChunk == null || !fromChunk.getHolders().contains(p)) {
          if (p.equals(this)) {
            return;
          }
          p.net().sendPacket(spawnThis);
        }
        if (!destChunk.getHolders().contains(p)) {
          p.net().sendPacket(new PlayOutDestroyEntities(destroy));
        }
      }
);
      player.updateChunks();
    }
 else {
      if (fromChunk != null) {
        fromChunk.getEntitySet().remove(this);
      }
      destChunk.getEntitySet().add(this);
      Stream.concat(fromChunk == null ? Stream.empty() : fromChunk.getHolders().stream(),destChunk.getHolders().stream()).distinct().forEach(p -> {
        if (fromChunk == null || !fromChunk.getHolders().contains(p)) {
          if (p.equals(this)) {
            return;
          }
          p.net().sendPacket(spawnThis);
        }
        if (!destChunk.getHolders().contains(p)) {
          p.net().sendPacket(new PlayOutDestroyEntities(destroy));
        }
      }
);
    }
  }
  Position delta=position.subtract(old);
  if (delta.getX() != 0 || delta.getY() != 0 || delta.getZ() != 0) {
    if (old.distanceSquared(position) > 16) {
      PlayOutTeleport packet=new PlayOutTeleport(this);
      RecipientSelector.whoCanSee(destChunk,null,packet);
    }
 else {
      if (Double.compare(old.getYaw(),position.getYaw()) == 0 && Double.compare(old.getPitch(),position.getPitch()) == 0) {
        PlayOutEntityRelativeMove packet=new PlayOutEntityRelativeMove(this,delta);
        RecipientSelector.whoCanSee(destChunk,this,packet);
      }
 else {
        PlayOutEntityLookAndRelativeMove lookAndRelativeMove=new PlayOutEntityLookAndRelativeMove(this,delta);
        PlayOutEntityHeadLook look=new PlayOutEntityHeadLook(this);
        RecipientSelector.whoCanSee(destChunk,this,lookAndRelativeMove,look);
      }
    }
  }
 else   if (Float.compare(old.getYaw(),position.getYaw()) != 0 || Float.compare(old.getPitch(),position.getPitch()) != 0) {
    PlayOutEntityLookAndRelativeMove lookAndRelativeMove=new PlayOutEntityLookAndRelativeMove(this,delta);
    PlayOutEntityHeadLook look=new PlayOutEntityHeadLook(this);
    RecipientSelector.whoCanSee(destChunk,this,lookAndRelativeMove,look);
  }
}