It's a 'thing' because all projectiles are invisible in the fog of war. It's not uncommon for projectiles to be 'invisible' because they were thrown from the fog of war and you were against a wall or simply walking towards the missile. The missile tends to become visible when it enters your little circle of view. If you're moving towards it and it's moving towards you, any bit of delay can cause it to seem invisible because it didn't have time to register that the spear went into your line of view. Normally, you can still see the Aura around it still, which is why Nidalee's Head Hunter skin is the most common one to be 'invisible' because the aura on it is much smaller and harder to see.
Now, Mundo's cleaver is about the same reason, but it's a rather small object when you actually look at it. It's hard to see as is, so when it magically becomes visible, you usually don't realize it until it's in your face.
I should also mention players with over 100 ping will experience this EVEN MORE because they have a LARGE DELAY from the minor bit of lag you are experiencing and also makes it harder to react because you not only don't see it coming, but the delay prevents you from properly dodging it.