On May 5, 2021, Diexin Liu was moving from Calgary, Alberta, to Toronto, Ontario, when he was stopped by a highway patrol officer near Swift Current, Saskatchewan. One thing led to another when he was arrested and the police found cannabis in the vehicle. Convicted by a trial judge who appeared to misunderstand the law governing expert witnesses the Saskatchewan Court of Appeal recently ruled that Mr. Liu’s trial was a miscarriage of justice.

1. Introduction

On October 2, 2025, the Saskatchewan Court of Appeal released the unanimous opinion indexed as R. v. Liu overturning Mr. Liu’s convictions for multiple cannabis-related and excise offences. (here) Writing for the three member panel, Justice J.M. Drennan held that the trial judge erred in admitting the expert opinion evidence of Sgt. R. Karaim and in limiting Mr. Liu’s ability to make full answer and defence. Although not specifically cited, the right to make full answer and defence falls under s. 7 of the Charter of Rights and Freedoms. The errors cumulatively resulted in a miscarriage of justice. The ruling presents an opportunity to review the well settled law governing expert opinion evidence in Canada.

2. Background

Mr. Liu’s vehicle was packed with “all of his worldly possessions” when the highway patrol officer stopped him to check his licence and registration. The vehicle also had tinted front windows the officer believed were unlawful. When speaking with Mr. Liu, the officer saw a package of “unstamped tobacco” in plain view in the vehicle. He called the Royal Canadian Mounted Police and Mr. Liu was arrested. The vehicle was searched incident to arrest and the officers found 5.094 kilograms of cannabis along with growing equipment. The vehicle also contained clothes, personal belongings and Mr. Liu’s identification documents.

The trial judge convicted Mr. Liu of three offences under the provincial Cannabis Act: (1) unlawfully possessing more than 30 grams of dried cannabis in a public place (possession count); (2) possession of cannabis for the purpose of distribution; and, (3) possession of cannabis for the purpose of sale (distribution and sale counts). (here) He was also convicted of possessing “unstamped cannabis” product contrary to the federal Excise Act. (here) He was sentenced to 13 months imprisonment concurrent on the distribution and sale counts, four months concurrent on the possession count and a fine of $10,525 on the count under the Excise Act. He appealed both conviction and sentence.

3. Expert Evidence

The core issue on appeal, involving the admissibility of Sgt. Karaim’s expert evidence, was threefold: (1) whether the trial judge erred in qualifying the officer as an expert by failing to apply the framework for admission of expert opinion evidence established in R. v. Mohan (here) and White Burgess Langille Inman v. Abbott and Haliburton Co. (here); (2) whether the trial judge improperly admitted the expert evidence on the ultimate issue of possession of cannabis for distribution and sale; and, (3) whether notice of the proposed area of qualification was given to Mr. Liu outside the timeline required under s. 657.3(3) of the Criminal Code. (here). Although Justice Drennan expressed concern “with how the trial judge performed his judicial function at every stage of the admission process” I will focus my following comments on aspects of the ruling that involve: (a) the qualifications voir dire; (b) the Mohan test; and, (c) evidence on the ultimate issue.

(a) Qualifications Voir Dire

Sgt. Karaim’s qualifications were not admitted by the defence and the trial judge did not conduct a voir dire to consider his qualifications. “Indeed,” Justice Drennan said, “he appeared to not understand the need for a voir dire in such a circumstance.” When asked by the prosecutor at the outset of the qualification process if a voir dire was required, the trial judge said, “I’m trying to think here. I don’t think we have to go into a voir dire, you just have to qualify him.” Although the trial judge ultimately considered Sgt. Karaim’s qualifications before allowing him to give opinion evidence, Justice Drennan found that his “somewhat cavalier approach to the qualification process creates genuine doubt as to his understanding of the method by which an expert is qualified to do so.”

(b) The Mohan Test

The two part test for the admission of expert evidence was set out in White Burgess. The first step requires a trial judge to determine whether the proposed evidence satisfies the four Mohan criteria of relevance, necessity, absence of an exclusionary rule and a properly qualified expert. The second step requires a trial judge to perform a gatekeeper role and consider whether the benefits of admitting the evidence outweigh the potential risks in doing so. In addition to failing to conduct a qualifications voir dire, the trial judge also failed to consider the Mohan test for admissibility. The trial judge qualified Sgt. Karaim on the basis of his curriculum vitae “without any reference to or analysis of the threshold requirements for admissibility.”

(c) The Ultimate Issue

The trial judge also failed to scrutinize aspects of Sgt. Karaim’s opinion that touched on the ultimate issue of whether Mr. Liu possessed the cannabis for the purposes of distribution and sale. Although there is no absolute rule prohibiting an expert from giving evidence on the ultimate issue, the Supreme Court of Canada held in Mohan that “even where the expert’s evidence is broadly necessary […] it should be assessed with special scrutiny as it approaches the ‘ultimate issue’.” Special scrutiny was required here because, at the outset of direct examination, Crown counsel orally requested an addendum to Sgt. Karaim’s qualifications that he also give evidence on the “sale of cannabis […] including the purpose for which cannabis is possessed in a particular case.” The mid-trial addendum went to the ultimate issue and deprived Mr. Liu of the notice required by s. 657.3(3) of the Code.

4. Miscarriage of Justice

A miscarriage of justice may result from any irregularity that renders a trial unfair or creates an appearance of trial unfairness and does not require a demonstration of actual prejudice. (here) A linear link is not required between an error and the verdict. In R. v. Khan, the Supreme Court of Canada held that, “if the reviewing court concludes that the error, whether procedural or substantive, led to a denial of a fair trial, the court may properly characterize the matter as one where there was a miscarriage of justice. In that case, no remedial provision is available and the appeal must be allowed.” (here) The “Mohan errors” coupled with “significant and unwarranted constraint” imposed on Mr. Liu’s right to cross-examine the officer undermined trial fairness. “The trial judge’s approach to Sgt. Karaim’s qualification and the admission of his opinion evidence effectively relieved the Crown of its burden in tendering and qualifying him as an expert, while also interfering with Mr. Liu’s ability to make full answer and defence,” Justce Drennan concluded.

5. Conclusion

In the result, Justice Drennan ordered a new trial on the distribution and sale counts. The sentence of four months imprisonment on the possession count was demonstrably unfit and a 12-month conditional discharge was substituted. The reasons took into account “the shifting legal and societal attitudes towards cannabis possession” and that Mr. Liu was in possession of a quantity that would have been legal if in his own home. The appeal from conviction under the Excise Act was dismissed. Under the excise statute, the absence of a stamp on the cannabis product indicated that applicable federal and provincial duties had not been paid.