Why does trace result differs between ClientSide and ServerSide?

[WeAthFolD]

I've been trying to work out something like guns, and I tried to use Minecraft's RayTraceBlocks function,and I've met with a strange difficulty.

class BulletManager{
        ....Other methods
public static void Shoot(ItemStack itemStack, EntityLiving entityPlayer, World worldObj, String particle){

	if(!(itemStack.getItem() instanceof WeaponGeneral))
		return;

	WeaponGeneral item = (WeaponGeneral) itemStack.getItem();
	InformationWeapon inf = item.getInformation(itemStack).getProperInf(worldObj);
	int mode = inf.mode;
	int damage = item.damage[mode];
	int offset = item.offset[mode]; //load Information of the specfic ItemStack
	MotionXYZ prem = new MotionXYZ(entityPlayer, 0).updateMotion(0.5D); //right before player
	MotionXYZ afterm = new MotionXYZ(entityPlayer, 0).updateMotion(300D); //end position

	worldObj.spawnParticle(particle , prem.posX, prem.posY, prem.posZ, prem.motionX, prem.motionY, prem.motionZ);


	//do rayTrace
	Vec3 pre = prem.asVec3(worldObj), after = afterm.asVec3(worldObj);
	MovingObjectPosition result = doTrace(worldObj, pre, after, entityPlayer);//Trace both blocks and entities
                //*******SERVER AND CLIENT BEHAVES DIFFERENTLY,WHY?QaQ

	if(result != null)
		System.out.println("RayTrace result: " + result.typeOfHit + ",isRemote: " + worldObj.isRemote);

	switch(result.typeOfHit){
	case ENTITY:
		doEntityCollision(result, itemStack, worldObj, entityPlayer, prem);
		if( worldObj.getBlockId(result.blockX,result.blockY,result.blockZ) > 0 ){
			doBlockCollision(result, itemStack, worldObj, entityPlayer, prem);
		}
		break;
	case TILE:
		doBlockCollision(result, itemStack, worldObj, entityPlayer, prem);
		break;

	default:
		break;
	}
	return;

}

private static MovingObjectPosition doTrace(World world, Vec3 pre, Vec3 after, EntityLiving player){

	MovingObjectPosition result;

	result = world.rayTraceBlocks_do(pre, after, true);
	MotionXYZ var0 = new MotionXYZ(player,0).updateMotion(0.5D);
	int blocksAway = 0;
	if(result != null){
		after = result.hitVec;
		blocksAway = (int) Math.round((after.xCoord - var0.posX)/var0.motionX);
	} else {
		after = new MotionXYZ(player, 0).asVec3(world);
		blocksAway = ENTITY_TRACE_RANGE;
	}
	System.out.println("BlocksAway : " + blocksAway);
	AxisAlignedBB bBox;
	List list;

	for(int i = 0; i < blocksAway; i++){
		bBox = AxisAlignedBB.getBoundingBox(var0.posX-BB_SIZE, var0.posY-BB_SIZE, var0.posZ-BB_SIZE,
				var0.posX+BB_SIZE, var0.posY+BB_SIZE, var0.posZ+BB_SIZE);
		list = world.getEntitiesWithinAABBExcludingEntity(player, bBox);
		if(list.size() >0){
			Entity var1;
			for(int j=0; j< list.size(); j++){
				var1 = (Entity) list.get(j);
				if(var1 instanceof EntityLiving){
					if(result == null)
						result = new MovingObjectPosition(var1);
					result.entityHit = var1;
					result.typeOfHit = EnumMovingObjectType.ENTITY;
				}
			}
		}
		var0.updateMotion(2*BB_SIZE);
	}
	return result;

}

public static void doEntityCollision(MovingObjectPosition result, ItemStack itemStack, World worldObj, EntityLiving player, MotionXYZ begin){

	if(result == null)
		return;
	if(!(result.entityHit instanceof EntityLiving))
		return;

	WeaponGeneral item = (WeaponGeneral) itemStack.getItem();
	InformationWeapon inf = item.getInformation(itemStack).getProperInf(worldObj);
	int mode = inf.mode;
	double pf = item.pushForce[mode];
	double dx = begin.motionX * pf, dy = begin.motionY * pf, dz = begin.motionZ * pf;
	EntityLiving mob = (EntityLiving) result.entityHit;
	System.out.println("Hit side: " + worldObj.isRemote + ", Mob Side: " + mob.worldObj.isRemote);
                //Attack Mob and add Velocity,which doesn't work everytime(ACTUALLY A LOT OF TIME)
	mob.attackEntityFrom(DamageSource.causeMobDamage(player), item.damage[mode]);
	mob.addVelocity(dx, dy, dz);

}

}

MotionXYZ is a class I created which contains PosXYZ and MotionXYZ information.

BTW,0 on the constructor is offset,which make MotionXYZ a random range.

The Shoot Method is called within Item class when holding Right mouse.

I expected the result to work out just fine,but it turns out that the collision result I get from Server and Client side is really different.

Once shot, I can see the mob being hit(Velocity changed),but it wasn't hurt and comes back to its origin position few seconds later.

on console I can see that Trace results between Client and Server are totally different,one on the CLIENT is ENTITY, another one on the SERVER is TILE.

I was really confused. How could it be possibly different when I use the same Vec3 at almost exact same tick?How do I fix this?If I can't , can I use other ways such as Trace only in CLIENT and hurt the mob on Serverside?

Sorry for my bad English, and thanks for your reading and answering my question

[opssemnik]

because client lies, these things must be done in the server to fix u need to use a if(!world.isRemote){ //before u call the code