Can a prosecutor improperly shift the burden of proof?

In People v. Hill, supra, 17 Cal.4th 800, the court held that the prosecutor did impermissibly shift the burden of proof. In her closing argument, the prosecutor stated, "'you have to have a reason for this doubt. There has to be some evidence on which to base a doubt, "" and "'[t]here must be some evidence from which there is a reason for a doubt. You can't say, well, one of the attorneys said so.'" (Id. at p. 831.) The court found these comments to be somewhat ambiguous. "[T]o the extent [the prosecutor] was claiming there must be some affirmative evidence demonstrating a reasonable doubt, she was mistaken as to the law, for the jury may simply not be persuaded by the prosecution's evidence. [Citation.] On the other hand, [the prosecutor] may simply have been exhorting the jury to consider the evidence presented, and not attorney argument, before making up its mind." (Id. at pp. 831-832.) The "question arguably [was] close," but the court concluded that it was reasonably likely that the jury understood the comments "to mean defendant had the burden of producing evidence to demonstrate a reasonable doubt of his guilt." (Id. at p. 832.)