It is a surprising thing to encounter a witness who, in all honesty, believes that the evidence they are giving or proposing to give is the truth, when it is far from it.
After the battle of Salamanca in the Peninsular war the Prince Regent was so enthused with Wellington’s victory there that he came to believe that he had been present. To accommodate the fact that nobody could corroborate this remarkable claim he suggested that he had been disguised as Major-General von Bock, who not only was there, but had led the charge of the King’s German Legion.