The story of the spear (Mao2) and the shield (Dun4)

This is the origin of a Chinese phrase "Mao2 Dun4" 矛盾, which was brought into the modern usage of the language through this story.

There was once a weaponry seller. He sold shields and swords and he often shouted to the crowd for buyers, saying "Look! This spear I have here can penetrate anything! Nothing in this world can escape the fate of destruction from this spear! Not even the sturdiest, hardest shield!"

Then, he would go on introducing his other weapons.
"Look! This shield I have is totally impenetrable! Nothing in this world can go through this shield! Even the sharpest, hardest spear!"

A man from the crowd, after listening, asked this seller.
"What if I use the spear you mentioned to stab that shield you sell? Would the spear successfully penetrate, or would the shield remain in one piece?"

The seller kept quiet in shame.

Hence, if you were to describe a situation similar to the story, you may use the phrase Mao2 Dun4 in Chinese.