During the Warring States (475—221 BC), a man from the State of Chu was selling spears and shields on the market.
He first held up a shield and praised, “My shields are incredibly sturdy — so sturdy that nothing can pierce through them.”
A moment later, he praised his spears, “My spears are incredibly sharp — so sharp that they can pierce through anything.”
Someone asked him, “What happens if we use your spear to pierce your shield?” The vendor could not answer.
The phrase “use one's spear to pierce one's own shield” has later become a common expression in describing incoherent statements or contradictory situations.
Source: “Han Fei Tzu” by Han Fei (circa 281—233 BC), a prince in the State of Han.