Mistake one: memorized answers
Interviewers can tell. A candidate who has practised "the answer" gives themselves away in the follow-up question. Reason out loud instead.
Mistake two: no opinion on trade-offs
"It depends" without follow-through is a red flag. Have a default, defend it, and know when you would change it.
Mistake three: no failure story
Everyone has shipped something broken. Candidates who cannot name one look inexperienced or dishonest.
Mistake four: no compliance vocabulary
At every major Canadian employer, PIPEDA, AIDA, and residency will come up. If you have never said those words out loud, practise.