A travelling salesman was advertising his wares. He was selling weapons and armour. One day he came to a village where a great soldier lived.You can see the story told in this walkthrough video, at timecode 4:50. In the near-future dystopian city of Europolis, Zoe comes across her friend Baruti, the election campaign manager for the Social Democrats, being threatened by the gangster's thugs. If (as in this walkthrough) Zoe chooses to intervene, this is the story which the gangster tells her as his warning.
"This shield," said the salesman "is so strong that nothing can pierce it. It will resist any impact. It will protect a warrior from all harm."
"This spear," said the salesman, "is so sharp that it will pierce any armour, killing your opponent instantly."
"How can it be," answered the great soldier, "that you have a shield that cannot be pierced and a spear that can pierce anything? The two cannot possibly co-exist."
So the salesman grabbed the shield and the spear. "It does not matter whether the two can or can not co-exist. m****rf****r", he said, running the great soldier through, killing him, "when I'm the one wielding both."