I'm not sure what is the point that you are making here. An actor who was given a fake bag of heroin as a prop which then turns out to be real is no more guilty

I'm not sure what is the point that you are making here. An actor who was given a fake bag of heroin as a prop which then turns out to be real is no more guilty than a courier moving a package that happens to contain drugs or guns or fake money or anything else - neither would be found guilty of posession. These are situational circumstances, and no prosecutor in the world would choose to prosecute those - but there is 0% chance you could get away with saying "oh I thought it was fake" if caught with CP on your phone.