The Illustrated Guide to Criminal Law Chapter 8: What Have You Done? Page 8: Concurrence
In this case, nobody could have predicted that spooking the pigeon would cause Simon to get struck on the head by a brick. So even though Joe’s act was the cause in fact, it was not the proximate cause, and so there is no actus reus.
Joe will be guilty only if he committed an actus reus—an act that caused the harm prohibited by that law—with the required mens rea.