In all but the rarest of cases, a trial judge has a duty to review with the jurors the issues to be resolved and the evidence they may consider in resolving those issues.  This process is known as the jury charge or jury instruction.
“The task of the trial judge is to explain the critical evidence and the law and relate them to the essential issues in plain, understandable language.”
R. v. Jack (1993), 88 Man. R. (2d) 93 (C.A.), at p. 102; aff’d 1994 CanLII 87 (SCC)
A formidable task, certainly, and, as one might expect, a task where errors may occur.  But the errors of the trial judge are not the only point of focus on an appeal.  Sometimes trial counsel’s failure to object to errors or deficiencies within a proposed jury charge has implications on appeal.
Where there is a Significant Divergence between the Accused’s Position on an Issue at Trial and his/her position on Appeal
Arguments by appellate counsel that fly in the face of positions taken by counsel at trial quite properly attract judicial skepticism and resistance.  The significance of counsel’s position at trial to the merits of the argument on appeal will depend on the nature of the argument advanced on appeal. 
Where it is alleged that an instruction to the jury was unclear, inadequate on a legal or evidentiary issue raised in the trial, or did not treat the position of the appellant at trial fairly or fully, counsel’s approval of the jury instruction and to a lesser extent his or her failure to object, will be a significant consideration on appeal.
Trial Counsel’s Failure to Request a Discretionary Instruction
Similarly, where an appellant argues that the trial judge failed to give an instruction that was discretionary, counsel’s failure to request the instruction at trial will be a significant consideration on appeal.
Trial Counsel Shouldn’t Keep his Objections to a Jury Charge in his/her Pocket in Order to Set up an Appeal
Counsel’s position at trial will become all the more significant on appeal if it appears that the position reflects a calculated tactical decision.
Where the Trial Judge Misstates the Law
Where, however, the argument is that the trial judge misstated the law in his or her instructions, counsel’s position at trial is of less significance on appeal.  If an error in law is demonstrated, counsel’s position at trial is potentially relevant to the application of the curative proviso.
However, counsel’s position cannot justify treating an instruction that is wrong in law as a correct statement of the law.  A misstatement of the law is no less an error in law because it was made with the full support and approval of trial counsel.  
See R. v. Austin (2006), 2006 CanLII 39077 (ON CA)
[FN: The section 686(1)(b)(iii) curative proviso is applied where there is no “reasonable possibility” that the verdict would have been different had the error at issue not been made]