For example I was in court last week for among other things the sentence of a man done for VAT fraud. He'd presented laughably bad forgeries to the inspectors when they put it to him that his claim for a refund was fraudulent, and he continued to say, even at sentence (so after being found guilty) that the documents were genuine. The judge had to more or less tell him to shut up because he was representing himself so he didn't have a lawyer to tell him.

For a jury this will have been easy, the defendant's claim is that these forgeries are genuine, but the jury can see for themselves (no need for an expert on document forgery) that these are childishly bad, incompetent. Is it in some sense technically "possible" that real documents look exactly like a toddler with learning difficulties tried to make them? Sure. But it's not "reasonable" to think that's the truth and so the jury were right to convict.