Fagan v Commissioner of Police of the Metropolis  1 QB 439 is a Criminal Law case concerning Mens Rea.
The defendant (Fagan) accidentally drove his car onto a policeman’s (V) foot and, when he became aware of this, he intentionally took his time moving the car in order to cause further pain. The problem was that when performing the Actus Reus (driving onto the foot) D lacked Mens Rea, but when he gained Mens Rea (intentionally causing pain) these acts were already complete.
The main issue in Fagan v Commissioner of Police of the Metropolis  1 QB 439:
Was this offence committed by omission ?
No. The Divisional Court agreed that assault cannot be committed by an omission. However, in this case, the crime was not an omission to move the car; rather, it constituted a continual act of battery. The offence was not complete until the moment Fagan realised that he had driven onto the foot of the officer and, in deciding not to cease this continuous act, formed an intent amounting to the mens rea for common assault. Because of the fact that both Actus Reus and Mens Rea were present, Fagan’s conviction of assault was upheld.