What is the test for exclusion of a witness from an examination for discovery?

The intent of the case law that allows for the exclusion of a witness from an examination for discovery is to prevent prejudice to one party that would arise from an opposing party being able to tailor their evidence to that of an opposing party. The discretion granted to the court to prevent such an injustice also enables the court to make orders with respect to the timing of the disclosure of documents. This is precisely what the court did in O’Halloran v. Clements, 2009 CarswellOnt 9726 (S.C.J.) when it permitted the defendants, whom the plaintiff alleged were the owners of a dog that bit him, to defer producing photographs of their dog until after the plaintiff provided a description of the dog in question, at his own examination.