Marvelous Mario’s Inc. et al. v. St. Paul Fire And Marine Insurance Co. provides authority for the principle that an amendment to plead discoverability is available at any time:
 The plaintiffs’ pleading is silent as to discoverability. Recognizing the gap in their pleading, the plaintiffs have moved for an order allowing them to amend their pleading to plead discoverability. St. Paul takes no position on the motion. Rule 26.01 of the Rules of Civil Procedure, R.R.O. 1990, Reg. 194allows for an amendment to be made at any time, even after the conclusion of trial: Hardy v. Herr, 1965 CanLII 225 (ON CA),  2 O.R. 801 (Ont. C.A.), at para. 2. Discoverability was an issue thoroughly canvassed at trial. I see no prejudice to St. Paul in granting the amendment. Accordingly, the plaintiffs’ motion to amend their statement of claim to plead discoverability is granted.
All of that said, it’s worth noting that the facts setting up a discovery argument are properly pleaded in reply.
In Lucky Star Developments Inc. v. ABSA Canada International, the Court of Appeal rejected the doubtful argument that because the basic limitation period applies to the commencement of proceedings, it does not apply to proceedings that have already been commenced, and therefore does not bar amendments under r. 26.01:
 In oral submissions, the appellant argued that s. 4 of the Limitations Act 2002, S.O. 2002, c. 24, Sched. B does not apply to proceedings that have already been commenced, and so does not bar amendments under r. 26.01. We disagree. As the court noted in Joseph, the rules must be read in light of the Act and its purpose in establishing a basic limitation period in s. 4. Amendments adding claims after the limitation period has expired constitute prejudice.
Though it’s plain this argument was bound to fail—it would mean there is no limitation of new claims asserted in already-commenced proceedings—it’s a symptom of the conceptual difficulties that arises from the language “proceeding in respect of a claim”.
The jurisprudence seems to have settled on “proceeding” having the same meaning as it does under the Rules. Rule 1.03 defines “proceeding” to include an action and an application, and the Court of Appeal has applied this definition to the term “proceeding” as used in the Limitations Act: see e.g. Giglio v. Peters, 2009 ONCA 681 at paras. 21-22 [“Giglio”]. See also Guillemette v. Doucet, 2007 ONCA 743 at para. 20.
Strictly applied, this means that s. 4 bars actions or applications commenced in respect of a claim. A proposed amendment to add a claim to an existing action is of course not a proposal to commence a new action. I’ve argued before that the solution to this tension is to abandon a narrow definition of “proceeding” and to define the commencement of a proceeding broadly enough to include amending a pleading to introduce a new claim.