An accused is constitutionally entitled to effective representation. Effective representation means reasonably competent representation. Counsel’s performance is measured without the benefit of hindsight and bearing in mind that the reasonable exercise of professional judgment will in many instances allow for different tactical decisions.
R. v. G.D.B., 2000 SCC 22 (CanLII),  1 S.C.R. 520, at paras. 27-28.
Was there a miscarriage of justice?
If an accused who receives ineffective representation at trial is convicted, the conviction must be quashed if the result is properly characterized as a miscarriage of justice.
A miscarriage of justice occurs if the ineffective representation either:
a. Sufficiently undermines the reliability of the verdict
· the reliability of a verdict is sufficiently undermined if the appeal court concludes that there is a reasonable probability that the verdict would have been different had the appellant received adequate legal representation.
b. Results in an unfair trial.
See R. v. Prebtani, 2008 ONCA 735 (CanLII), at para. 4;
R. v. Joanisse, 1995 CanLII 3507 (ON CA),  O.J. No. 2883, at paras. 74-80 (C.A.), leave to appeal to SCC refused,  S.C.C.A. No. 347.
Failure of trial counsel to bring a 276 application
Section 276 of the Criminal Code requires that before evidence can be adduced of sexual activity other than the activity alleged in the charge, counsel must apply for and obtain an order permitting the eliciting of that evidence.
In some instances, appellate counsel will be able to establish that the failure of trial counsel to bring a 276 application sufficiently undermined the reliability of the verdict.
See for example R. v. Walendzewicz, 2018 ONCA 103, at para. 10.
However, the failure of trial counsel to even attempt to bring a s. 276 application may itself be enough to establish ineffective representation.
R. v. R.S., 2016 ONCA 655, 341 C.C.C. (3d) 530;
See also R. v. Walendzewicz, 2018 ONCA 103, at para. 10 (obiter).