When and where is incompetence in committing a crime is an excuse? I mean, should we give an unwitting incentive to get it right? “Officer, yes, I didn’t really get the loot in that kidnapping, and the victim escaped, so i should not be charged with a crime”. There are whole categories of attempted crime-murder, rape, other forms of mayhem. The idea is that you shouldn’t even try to commit them. We are not going to trust to luck that you are too incompetent, or the victim too lucky.