When is an unannounced forced entry into an occupied dwelling reasonable? In R v Cornell  SCC 31 the majority held there were sufficient circumstances to make such a search reasonable. These cases will usually be fact-specific, as here, and no new principles of law were established.
The majority (McLachlin C.J. and Charron, Rothstein and Cromwell J, joint judgment delivered by Cromwell J) stated the general principles:
"The only issue is whether the lawfully authorized search was conducted reasonably. Except in exigent circumstances, police officers must make an announcement before forcing entry into a dwelling house. Ordinarily, they should give: (1) notice of presence by knocking or ringing a door bell; (2) notice of authority, by identifying themselves as law enforcement officers; and (3) notice of purpose, by stating a lawful reason for entry. While the "knock and announce" principle is not absolute, where the police depart from it, there is an onus on them to explain why they thought it necessary to do so. If challenged, the Crown must lay an evidentiary framework to support the conclusion that the police had reasonable grounds to be concerned about the possibility of harm to themselves or occupants or about the destruction of evidence. The police must be allowed a certain amount of latitude in the manner in which they decide to enter premises and, in assessing that decision, the police must be judged by what was, or should reasonably have been, known to them at the time. On appellate review, the trial judge's assessment of the evidence and findings of fact must be accorded substantial deference."
In another fact-specific case decided the same day, the Court by the same majority (but here delivered by Charron J) held that the law may require a trial judge to put to the jury matters of law not covered by counsel (even by agreement between counsel), with the result that a basis for conviction which was not the subject of addresses to the jury by counsel may properly be considered and relied on in deliberation of the verdict: R v Pickton  SCC 32 This is really because the trial must be in accordance with the law, subject to fairness. Here the jury had been asked to acquit based on a factual doubt (as to his being a principal), but that doubt would not, on a wider view of routes to guilt (secondary liability), have exculpated him.
The position in Canada is that, even if the Crown has consistently advanced only one theory of guilt, its case is "a moving target": R v Rose, 1998 CanLII 768 (S.C.C.),  3 S.C.R. 262. The issue will be whether a change in stance gives rise to unfairness to the accused. It was significant here that "the defence theory itself put the participation of others at issue" (21, the majority's emphasis), and this meant that throughout the trial defence counsel were aware that secondary participation was in issue. The minority considered that the judge's instruction on secondary liability was inadequate.