It arises a lot in common law and it has arisen over the years with consent in sexual offences. There's a case called Pappajohn, which permitted a defence to an allegation of sexual assault if a person had an honest but mistaken belief in consent. Legislation dealt with that and has significantly altered that framework.
The difference between sexual assault and murder is that murder has a mens rea requirement that is entirely subjective. It's one of the very few criminal offences that requires the highest level of mens rea, which is a subjective fault. In other words, you know what you're doing at the time is wrong.
With this legislation, as drafted, if somebody makes an honest mistake, they could still fall outside of the exemption. The exemption, we have to remember, as I said at the beginning of my remarks, is to what is otherwise in all other circumstances the crime of murder. That's the problem.