Fagan v Metropolitan Police Commissioner is a leading case that confirms the need for concurrence (or coincidence) of actus reus (Latin for "guilty act") and mens rea (Latin for "guilty mind") in most offences of the criminal law of England and Wales.
In accordance with the directions, Fagan backed his car up, accidentally rolling it onto the foot of the officer.
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.