What constitutes an error of law subject to a standard of correctness?

Where an error involves the application of a legal standard to a set of facts, a failure to consider a required element of a legal test, or similar error in principle, this will amount to an error of law subject to a standard of correctness. Housen v. Nicolaisen, supra, at p. 595

An appellate court must take a more cautious approach where, for example, it is said that a trial judge erred in law in his or her determination of negligence, as it is often difficult to extricate the legal questions from the factual. Where the legal principle is thus not readily extricable, the matter of mixed law and fact under review is subject to the more stringent standard, and shall not be overturned absent palpable and overriding error. The general rule is that if the issue under appeal involves the trial judge’s interpretation of the evidence as a whole, it should not be overturned absent palpable and overriding error. Housen v. Nicolaisen, ibid.