His spear against his own shield -- Be self-contradictory

Once upon a time, a man of the State of Chu had a spear and a shield for sale in the streets. Spears! Shields! First class spears and strong shields!

He was loud in praises of his shield. "Look, my shield is so strong that nothing in the world can pierce it through, no matter how hard and sharp it may be."

With a proud expression on his face, the man put down his shield and picked up a spear. He also sang praises of his spears, "My spear is so strong that it can pierce through anything. It is the best under the sky. Come on, buy my spears and shields."

Then a passerby asked him, "What would happen if your spear is used to pierce your shield?"

Then… erh… the seller was speechless and the crowd burst into laughter. It is impossible for an impenetrable shield to coexist with a spear that finds nothing impenetrable.

The idiom is from this story. Interestingly, the Chinese characters  for spear and for shield together form a compound , meaning to contradict, contradiction or contradictory.