Fagan v Metropolitan Police Comr
Fagan v Metropolitan Police Commissioner  1 QB 439,  3 All ER 442,  3 WLR 1120, 52 Cr App R 700, DC is a leading case that illustrates the requirement of concurrence (or coincidence) of actus reus (Latin for "guilty act") and mens rea (Latin for "guilty mind") in order to establish an offence under the criminal law of England and Wales.
The defendant, Mr. Fagan, was in his car when a police officer approached him and told him to move his car. In accordance with the directions, Fagan backed his car up, accidentally rolling it onto the foot of the officer. When the officer yelled at him to move his car off his foot, he cursed back at him, told him to wait, and refused to move.
At trial, Fagan was convicted of "Assaulting a constable in execution of his duties". Fagan appealed on the grounds that there can be no offence in omitting to act and that the act of driving onto the constable's foot was done completely by accident so there was no mens rea.
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. Since both mens rea and actus reus were present, an assault had been committed, and Fagan's conviction was upheld.