Professional consultants are imperfect just the same as everybody; however, when a professional consultant makes a mistake, the error or omission may result in death or injury, damage to property, serious financial impact, or sometimes even a combination of these consequences. Of course, the type professional can vary and thus the harm that results from a mistake also varies depending on the type of professional involved; a mistake by a doctor may cause death or injury; a mistake by an engineer may cause a bridge to collapse involving property damage, and a mistake by an accountant could cause tax reassessments resulting in retroactive interest due among other monetary losses.
Required Evidence, expert opinion report
In most circumstances, the opinion of a subject expert will be required to demonstrate that the standard of care was breached by the professional unless the allegations of professional negligence relate to a non-technical or egregious error. This was stated in Guindon v. Dolson, 2012 ONSC 1968:
 Mr. Bennett, counsel for Mr. Dolson, ably argued that the lack of expert evidence in this case is fatal to the Claim. I agree. Expert evidence is required to establish the requisite standard of care in professional negligence cases, including those against lawyers, except in (i) extraordinary cases involving non-technical matters or (ii) cases involving plainly egregious conduct. This Court draws that conclusion on the authority of jurisprudence from the Court of Appeal for Ontario including the decisions in Gauvreau v. Paci, 1996 CarswellOnt 2661 and Krawchuk v. Scherbak, 2011 CarswellOnt 3015.
As above, there are also cases where the error or omission is plain and obvious and more a basic function of the professional rather than a function that requires a technical analysis and understanding of various aspects of the professional service. Accordingly, mistakes such as basic adminstrative errors may be accepted by the court without requiring expert testimony that such mistakes breach the standard of care. This was stated well in Hill v. Queensbury Strategies Inc., 2014 CanLII 45416 at page 16, where it is said:
Considering the unique evidence and facts of this case, I do not need expert evidence to conclude that Skakie was negligent and in breach of the standard of care applicable to registered mutual fund sales representatives. His actions do not involve technical and complex details of the mutual fund industry for which I need assistance in determining whether there was a breach of the standard of care. He breached the standard of care in the following ways:
(a) he was told by the Plaintiff to change her mailing address but never took the steps necessary to ensure that the address was changed in the company records;
(b) he failed to take the steps necessary to ensure the Plaintiff understood at all material times the amount of B2B loans, understood the interest rate being charged, and understood the actual position of her account in terms of whether she was in a profit or loss position;
(c) he failed to obtain correct information on the B2B interest rate when opening the account;
(d) he failed to ensure that the Plaintiff understood how the monthly payments were reduced to $150.00. He failed to ensure that the Plaintiff understood that there was not an adjustment of interest rate, but rather a reduction in the principle of the loan;
(e) he never took the steps necessary to ensure that the Plaintiff had internet access to her account.
After the Plaintiff meets the burden of proving that the professional failed to meet the proper standard of care, proof of causation of loss follows the 'but for' test. This was simply stated in Hill at page 20 as:
Causation is determined after the court has found a breach of a duty of care. The basic test for determining causation remains the “but for” test – the plaintiff must show on the balance of probabilities that, but for the negligence of the defendant, the injury would not have occurred.
Just as there are many types of professionals whose mistakes could cause harm or injury, there are actual examples including cases against:
Property Inspection Services
Insurance Agents or Broker
Legal Advisors (lawyers or paralegals)
Guindon v. Dolson, 2012 ONSC 1968
Hill v. Queensbury Strategies Inc., 2014 CanLII 45416