The general principles and the operation of rectification were explained in Canada (Attorney General) v. Fairmont Hotels Inc., 2016 SCC 56 :
If by mistake a legal instrument does not accord with the true agreement it was intended to record — because a term has been omitted, an unwanted term included, or a term incorrectly expresses the parties’ agreement — a court may exercise its equitable jurisdiction to rectify the instrument so as to make it accord with the parties’ true agreement.
Alternatively put, rectification allows a court to achieve correspondence between the parties’ agreement and the substance of a legal instrument intended to record that agreement, when there is a discrepancy between the two. Its purpose is to give effect to the parties’ true intentions, rather than to an erroneous transcription of those true intentions
Because rectification allows courts to rewrite what the parties had originally intended to be the final expression of their agreement, it is “a potent remedy” be used “with great caution”, since a “relaxed approach to rectification as a substitute for due diligence at the time a document is signed would undermine the confidence of the commercial world in written contracts”:
Rectification is limited solely to cases where a written instrument has incorrectly recorded the parties’ antecedent agreement . It is not concerned with mistakes merely in the making of that antecedent agreement: E. Peel, The Law of Contract (14th ed. 2015), at para. 8-059; Mackenzie v. Coulson (1869), L.R. 8 Eq. 368, at p. 375 (“Courts of Equity do not rectify contracts; they may and do rectify instruments”).
In short, rectification is unavailable where the basis for seeking it is that one or both of the parties wish to amend not the instrument recording their agreement, but the agreement itself.
The court’s task in a rectification case is . . . to restore the parties to their original bargain, not to rectify a belatedly recognized error of judgment by one party or the other”.
Two types of error may support a grant of rectification:
1) The first arises when both parties subscribe to an instrument under a common mistake that it accurately records the terms of their antecedent agreement. In such a case, an order for rectification is predicated upon the applicant showing that the parties had reached a prior agreement whose terms are definite and ascertainable;
2) that the agreement was still effective when the instrument was executed; that the instrument fails to record accurately that prior agreement; and that, if rectified as proposed, the instrument would carry out the agreement: …
Rectification is limited solely to cases where a written instrument has incorrectly recorded the parties’ antecedent agreement”
The test for rectification requires courts to assess the true intention of the parties:
In order for rectification to be available, it is necessary to identify a “true agreement” which precedes (and is not accurately recorded by) the written instrument. Such an agreement may itself be contained in a written instrument; but it may be oral, and need not itself have contractual force.
