Chinese Idiom：自相矛盾（zì xiāng máo dùn）

A man of the state of Chu (chǔ guó 楚国) had a spear and a shield for sale. He was loud in praises of his shield.
"My shield is so strong that nothing can pierce it through."
He also sang praises of his spear.
"My spear is so strong that it can pierce through anything."
"What would happen," he was asked, "if your spear is used to pierce your shield?"
It is impossible for an impenetrable shield to coexist with a spear that finds nothing impenetrable.

楚国有个卖兵器的人，在市场上卖矛和盾。为了让人家愿意买他的货，他先举起盾向人们夸口道：“你们看，我的盾是世上最坚固的盾，任何锋利的东西都不能刺穿它。”接着又举起他的矛，向人吹嘘说：“你们再看看我的矛，它锋利无比，无坚不摧，无论多么坚硬的盾，都挡不住它，一刺就穿！”人群中有人问道：“依你的说法，你的矛无论怎样坚硬的盾都能刺穿，而你的盾又是无论多么锋利的矛也不能把它刺穿。那就拿你的矛来刺你的盾吧，看看结果怎么样？”卖兵器的人听了张口结舌，无从回答，只好拿着矛和盾走了。“自相矛盾”用来比喻做事、说话前后不一致，自相抵触。

自相矛盾 (zì xiāng máo dùn)
【翻译】 self-contradiction.
【释义】矛，进攻敌人的刺击武器；盾，保护自己的盾牌。比喻自己说话做事前后抵触。
【例句】你刚才的发言前后自相矛盾，别人不知道该怎么理解。
【近义词】格格不入、扞格不入
【反义词】自圆其说、无懈可击、天衣无缝、言行一致