Impossibility
If X incites Y to kill Z but, unknown to both of them at the time, Z had already died, it would be impossible to kill Z and so no crime of incitement would have been committed. Apart from simple situations such as this, the current law is difficult. R v Fitzmaurice allows the impossibility defence, but its scope is quite limited. X planned to collect a reward from a security firm by informing the police of the existence of a conspiracy to rob a security van. He recruited the defendant who thought he was engaging men for this robbery. Subsequently, the conspirators were arrested by the police. The Court of Appeal held that the test was to decide what sort of conduct was incited, attempted or the subject of a conspiracy. If the evidence shows incitement in general terms, e.g. to rob a security van, this is always possible, whereas if the subsequent agreement relates to a specific but fictitious crime, there might be an acquittal. In DPP v Armstrong Crim LR 379, 1999 EWHC 270 (QB) it was held that impossibility of the commission of the offence incited was irrelevant to guilt.
Read more about this topic: Incitement