In theory, if the actus reus does not hold concurrence in point of time with the mens rea then no crime has been committed.
Whereas in the law of agency, a principal may retrospectively adopt a transaction as if the agent had originally been authorised to conclude an agreement with a third party ("ratification" of the agent's decision), and so acquires liability under that agreement, an alleged criminal cannot retrospectively adopt an actus reus and acquire guilt.
[1] Because of the steel toe cap in his boot, the policeman's foot was not in actual danger, but the Divisional Court held that this could constitute a common assault.
A different way of justifying liability in this type of situation would be to consider an omission at the point in time that the mens rea is formed.
[It is fair to exclude liability when B's fear is entirely unreasonable given A's behaviour because B's self-induced injury will break the chain of causation].
This latter example raises a separate issue which is that it is sufficient to base a conviction on the presence of mens rea at some time during the occurrence of the events comprising the single transaction.
For example, suppose that A begins to strangle B and, believing B to be dead, abandons the "body" in nearby woods where B dies of exposure.
A will still be convicted of the homicide even though the relevant behaviour of abandoning the body was not accompanied by a mens rea.
And for the sake of completeness, if A commits an offence with an actus reus and a mens rea, it will not affect liability that A subsequently repents the crime and effects restitution.