R. v. Cadeddu, 2013 ONCA 729:
(c) Principles of Party Liability
 Section 21 of the Code is set out at para. 28, above. Subsection 21(1) deals with principals, aiding and abetting. Subsection 21(2) imposes a broader liability. It applies where one person commits an offence beyond the one with which the parties had originally planned to assist one another. It imposes liability on the other person if that person knew or ought to have known that the offence committed would be a probable consequence of carrying out the original common unlawful purpose. The Supreme Court explained in R. v. Logan,  2 S.C.R. 731, at p. 746, that the objective of s. 21(2) “is to deter joint criminal enterprises and to encourage persons who do participate to ensure that their accomplices do not commit offences beyond the planned unlawful purpose.”
 The distinctions between ss. 21(1) and 21(2) were described by Watt J.A. in R. v. Simon, 2010 ONCA 754, 104 O.R. (3d) 340, leave to appeal to S.C.C. refused,  S.C.C.A. No. 459, at paras. 39-43:
Persons who participate in the offence actually committed, whether as a principal, an aider or an abettor, have their liability determined under s. 21(1).
Section 21(2) extends liability for crime in two respects. The first has to do with the persons whose participation in an unlawful enterprise may attract liability. And the second relates to the offence for which participants in an unlawful criminal enterprise may be held liable: R. v. Simpson,  1 S.C.R. 3, at p. 15; R. v. Turkiewicz, Barrow and MacNamara (1979), 50 C.C.C. (2d) 406 (Ont. C.A.), at p. 409; R. v. Howard and Trudel (1983), 3 C.C.C. (3d) 399 (Ont. C.A.), at p. 408.
The persons to whom s. 21(2) extends liability are those whose participation in the offence actually committed would not be captured by s. 21(1). These persons have participated in a prior unlawful enterprise with others and either knew or, in most cases at least, should have known that one (or more) of the other participants in the original enterprise would likely commit the offence charged in pursuing their original purpose.
The offence to which s. 21(2) extends liability is not the original “unlawful purpose” to which the subsection refers. The “offence” of s. 21(2) is a different crime, one that a participant in the original “unlawful purpose” commits in carrying out that original purpose. And so it is that we sometimes say that s. 21(2) extends liability to those engaged in one unlawful purpose to incidental or collateral crimes: crimes committed by any participant (in the original purpose) in carrying out the original purpose that the other knew or should have known would likely be committed in pursuing the original purpose.
Under s. 21(2), the liability of a party to a common unlawful purpose for an incidental crime committed by another participant requires proof of the party
's participation in the original unlawful purpose,
the commission of the incidental crime by another participant and the required
degree of foresight of the likelihood that the incidental crime will be
committed. Consistent with general principle, each of these essential elements,
earlier described as “agreement”, “offence” and “knowledge”, must be supported
by an adequate evidentiary record to warrant submission of this basis of
liability to the jury. What we require is some evidence on the basis of which a
reasonable jury, properly instructed, could make the findings of fact necessary
to establish each element of this mode of participation: R.
v. Cinous,  2 S.C.R. 3,
at para. 83; R. v. Isaac,  1 S.C.R. 74,
at p. 81; R. v. Sparrow (1979), 51 C.C.C. (2d) 443 (Ont. C.A.), at p. 458. [Emphasis in
 This passage makes the important point that the scope of s. 21(2) is broader than s. 21(1). It extends liability to those who would not be found responsible as aiders or abettors and it extends responsibility for offences other than the offence in which they participated, if they had the requisite degree of foresight in respect of the commission of the offence charged: R. v. Ferrari, 2012 ONCA 399, 287 C.C.C. (3d) 503, at para. 62, per Rosenberg J.A., referring to R. v. Jackson (1991), 68 C.C.C. (3d) 385 (Ont.
), at p.
 As noted in Simon, at para. 43, there are three elements of liability under s. 21(2):
(a) agreement: participation of the party in a common unlawful purpose;
(b) offence: commission of an incidental and different crime by another participant; and
(c) knowledge: foreseeability of the likelihood of the incidental crime being committed.
A trial judge instructing a jury on this route to liability is required to explain these elements.
 I will review each of these elements and discuss the instruction the jury should have been given with respect to each. Before doing so, it is worth noting that the wording of s. 21(2) is – on its face – ambiguous. The “unlawful purpose” could refer either to the offence with which the accused is charged or to a different offence. The Supreme Court identified this ambiguity in R v. Simpson,  1 S.C.R. 3, at p. 11. Justice McIntyre, who gave the judgment of the court, clarified that the unlawful purpose must be different from the offence charged. He stated, at p. 15:
While acknowledging that the words of s. 21 of the Criminal Code could be read to support either contention… I am of the opinion that … the unlawful purpose mentioned in s. 21(2) must be different from the offence which is actually charged. The "unlawful purpose" and "the offence" committed in the course of the pursuit of the unlawful purpose are different. The two subsections of s. 21 deal with different circumstances. Subsection (1) applies to make everyone a party to an offence who commits it or who aids and abets in its commission. Subsection (2) covers the case where, in the absence of aiding and abetting, a person may become a party to an offence committed by another which he knew or ought to have known was a probable consequence of carrying out an unlawful purpose in common with the actual perpetrator. [Citations omitted.]
 As I have noted, the Crown concedes that this distinction was not explained to the jury.