Several classic reasoning jokes. Don't attach the answer.

During the heyday of the Northern Song Dynasty, Jiang Zhi, a magistrate of Renshou County, Sichuan Province, accepted a land property lawsuit shortly after taking office.

Zhang, the plaintiff, is a small official who specializes in collecting taxes, and sued his neighbor Wang for occupying 20 mu of his fertile land for no reason.

Wang argued, "There is no such thing. This 20 mu of land was left by my grandfather. Last year, Zhang came to my house to collect taxes, saying that if the inheritance was placed in his name, he could not pay taxes and refused to obey the corvee. I was worried about not paying taxes, so I agreed. At that time, we agreed that my land and real estate should be allocated to him in writing, but in fact the land and real estate still belong to me. "

Zhang said: "10 years ago, the Wangs offered to sell me 20 mu of land for emergency use, as evidenced by written evidence."

The magistrate of a county took the written evidence and looked at it carefully. This folded paper is written on white rice paper. The paper has turned yellow, and the edges of the paper are worn a lot, which seems to be a long time ago. The magistrate of a county folded the written evidence, unfolded it and folded it again.

Suddenly, his eyes lit up, he patted the gavel and shouted, "How dare you be bold and unruly, how dare you forge a written evidence, cheat the county, and not tell the truth!" "

What loopholes did the magistrate find in the written evidence?