Im making an enchantement that reflects arrows. My code works, but its causing a rather strange bug. When an arrow is deflected, it appears to fall to the ground, then teleport to where it should have impacted. My code:

 

private static void reflect(Entity projectile, EntityPlayer player)
{
  Vec3d vector = new Vec3d(-projectile.motionX,-projectile.motionY,-projectile.motionZ);
  vector = vector.normalize();

  projectile.motionX = vector.x;
  projectile.motionY = vector.y;
  projectile.motionZ = vector.z;
  System.out.println("checkpoint 3");
}

 

what could be causing this rather strange effect?

IIRC this happens with vanilla arrows too sometimes. Arrows in my experience are (one of) the most hacked together things in minecraft as they have special handling in many other classes.

 

In the arrow code you can see that vanilla applies the deflection through motion *= -1 (it might be -0.1, I can’t check right now) instead of using a vector. This is probably unrelated to your problem, but worth a try IMO

Not having seen all your code, your symptoms (rubber banding) sound rather like you're updating the entity's motion on the server, but not the client.  The server's position thus gets out of sync with the client's idea of the position, and you see the "teleport" effect when the server finally sync's up.

 

Any changes to an entity's position should be done simultaneously on both server and client.  Are you only calling your reflect() method on the server?