The plaintiffs in Paul v. Sasso were shareholders in a company that owned and operated a hotel. They brought a claim against the company and its majority shareholder. The central issue dealt with the valuation of shares. While the plaintiffs had retained an expert, the plaintiffs also decided to provide their own “report” even though neither was a qualified assessor. The trial judge found their report to be “not helpful” and was a waste of the court’s time. Unfortunately, the plaintiffs’ expert made reference to various aspects of the plaintiffs’ “findings” in their “report”. This, the trial judge found, “brought [the plaintiffs’ expert’s] objectivity into serious question”. In the end, the trial judge preferred the evidence of the defendant’s expert valuator.
The plaintiffs subsequently sued their expert witness alleging that they were lay people and did not understand that their “report” would invalidate the testimony of their expert. They alleged that their expert was negligent and breached the terms of their engagement as well as the terms of the Canadian Uniform Standards of Professional Appraisal Practice in failing to provide an unbiased and professional technical review report. The expert brought a motion for summary judgment to dismiss the claim and sought an order granting his counterclaim against the plaintiffs for unpaid fees.
Expert Witness Immunity
Justice Dunphy granted the summary judgment motion in part and dismissed the negligence claim against the expert witness relying on the long held fundamental principle that witnesses and parties are entitled to absolute immunity from subsequent liability for their testimony in judicial proceedings, as the “proper administration of justice requires the full and free participation of witnesses unhindered by fear of retaliatory suits”.
Justice Dunphy concluded that there “was no reason why the privilege should be confined to adverse witnesses” and that the harm that could follow from allowing parties to pursue their own experts for alleged breaches was amply illustrated by the facts of this case. The trial judge made a binding determination of the value of the shares and that determination had not been set aside on appeal. The plaintiffs’ case amounted to a de facto appeal of the decision as the damages sought were based on the value of the shares not found by the trial judge:
"The trial judge had the issue of value before her and weighed the testimony of all of the witnesses. Her determinations bind the plaintiffs and cannot be questioned through the back door by a subsequent civil suit. The principle of finality strongly supports the application of the common law immunity in this case. The question of what the result would have been if [the expert] had been accepted as fully independent can neither be asked nor answered in another proceeding."
However, Justice Dunphy went on to find that this did not mean that the alleged breach of contract and duty were not available as a defence to the counterclaim for unpaid fees:
"I am of the view that witness immunity can properly be used as a shield by [the expert] to avert liability on the plaintiff’s claim but cannot be used as a sword by [the expert] to preclude [the plaintiffs] from defending [the] counterclaim on the basis of the alleged breaches of contract and negligence."[emphasis added]
It was not possible, however, to resolve the factual disputes necessary to dispose of the significant issues without a trial of the counterclaim. While the expert was successful in his summary judgment motion in dismissing the negligence claim against him, his motion for summary judgement against the plaintiffs for unpaid fees was dismissed and a trial was ordered.
For now it appears that expert witnesses are likely entitled to immunity from negligence claims brought by the very party that hired them. However, there do not appear to be any appellate decisions dealing with this question, and the law may be far from settled. As noted by the court in Robinson v. Corporation of the City of Ottawa:
"There are no decisions of the Court of Appeal for Ontario, or any other Canadian court which has decided the issue of whether witness immunity should be extended to prevent a party from suing his or her own expert witness in negligence or for breach of contract, based on the opinion evidence given in a Court proceeding. Given the absence of any Canadian authority directly on point, I conclude that the law is unsettled on this issue."[emphasis added] 
Unlike in Paul v. Sasso, the Court in Robinson determined that the issue of whether you could sue your own expert witness could not be determined on a summary judgment motion and a trial was ordered. However, no trial decision appears to be reported.
Nevertheless, the law is settled in the United Kingdom. In the case of Jones v. Kaney,  UKSC 13 the appellant sued his own expert for negligently signing a statement of matters agreed to with the expert instructed by the opposing side, resulting in a reduced settlement. The majority of the Supreme Court decided that expert witnesses are not immune from claims in tort or contract for matters connected with their participation in legal proceedings, though they remain entitled to absolute privilege in respect of claims in defamation.
So far, this removal of expert witness immunity for a party’s own expert has not found its way to Canada.
 2016 ONSC 7488 (“Paul”)
 Paul v. 1433295 Ontario Limited, 2013 ONSC 7002 (CanLII) at paras.55- 56
 Paul at para. 16, citing Reynolds v. Kingston (Police Services Board), 2007 ONCA 166 (CanLII) at para. 14
 Paul at paras.17-18
 Paul at para. 18
 Paul at para. 34
 Robinson v. Ottawa, 2009 CanLII 1660 (ONSC) at para. 53