It is improper to ask a witness to comment on the credibility of another witness

 R. v. C.S., 2012 ONCA 403 deals with a problem repeatedly seen, a question asking a witness to comment on the credibility of another witness.  Such a question is improper:

[9]          We need not address the numerous other arguments advanced on behalf of the appellant, with one exception. It was improper of the trial judge to ask the appellant if he knew of any reason why the complainant would make up the things she testified about.

[10]       This court has held repeatedly that it is an error to call upon an accused to comment on the credibility of his or her accuser. See, for example, R. v. L.L., [2009] O.J. No. 2029 (C.A.) at para. 15; R. v. Rose, 53 O.R. (3d) 417, at para. 27.  As Simmons J.A. noted in L.L., at para. 16, such a question creates a serious risk of shifting the burden of proof to the accused instead of focusing on the central issue of whether the Crown has proved the case against the accused beyond a reasonable doubt.