Impenetrable Shield and All-piercing Spear

An armorer of London boldly claims to make the best spears and shields.

"My shields are so strong; they cannot be penetrated by any weapon," he said.
"My spears are so sharp; they can pierce any shield," he further said.

A man asks, "If your spear is thrown at your shield, what then?"
The armorer had no reply.

By logic, both an impenetrable shield and an all-piercing spear can not exist at the same time.

Moral of the story: Careful about the logic in any story.