What will really happen? -
A spear sharper and stronger than anything can pierce through ANY material.
A shield that can block even the sharpest things.

What if you try to pierce through the shield using that spear?

Same concept as an unstoppable force meeting an immovable object, which is to say, they would be the exception to each other.

The spear can pierce any material, so the shield will indeed be pierced, but only slightly.  Because the shield will be holding up it's end too, and blocking the spear from going too deep through the shield's surface.  

So on the one hand the spear went through, but not as much as against others.   On the other, the shield did block the attack, just not as well as it would've against others.