Imagine that you (or a loved one) were being tried for a criminal offense, and you thought that you (or your loved one) were legally insane at the at time of the crime. You might not always have been insane; after all, even the most nutty of people tend to have periods of lucidity. But you thought that, at the relevant time, the perpetrator of the charged offense was incapable of telling right from wrong, and hence was legally insane.
Not imagine further that the trial court gave the jury the following jury instruction, which comes from CALCRIM No. 3450: "If you conclude that at times the defendant was legally sane and other times the defendant was insane, you must assume that he was legally sane when he committed the crime." How would you feel about that? An accurate instruction? Don't forget, by the way, that the defendant has the burden of proving insanity. Doesn't this instruction tell the jury that as long as the defendant was ever legally sane, they gotta find him guilty; e.g., that they "must assume that he was legally sane when he committed the crime"?
Justice Hull concludes that the instruction is potentially misleading, but nonetheless doesn't require reversal because no reasonable juror would be misled by it. I agree with the former conclusion. In spades.