Functus officio, a branch of the doctrine of res judicata prevents the re-opening of a matter before the same court, tribunal or other statutory actor which rendered the final decision in the absence of statutory authority. There are many exceptions. For instance, where a statute authorizes variations of the original decision, the decision maker may revisit his or her previous decision. Common examples include competency hearings, parole board hearings and family law proceedings.
In the Canadian case of Chandler v Alberta Association of Architects, Sopinka J. wrote in relation to the principle of functus officio: "The general rule (is) that a final decision of a court cannot be reopened.... "The rule applied only after the formal judgment had been drawn up, issued and entered, and was subject to two exceptions: where there had been a slip in drawing it up, and where there was an error in expressing the manifest intention of the court."
However, there is an important difference between "res judicata" and "functus officio". "Res judicata" refers to the end of a case; "functus officio" refers to the expiration of an office. "Functus officio" derives from "fungere": to perform, end, expire. It is cognate to the ordinary English word "defunct". "Res judicata" means "the thing has been decided". "Res judicata" derives from the principle "interest reipublicae ut sit finis litium" (it is for the good of the commonwealth that there be a terminus to litigation.
Canadian doctrinal and judicial writing on functus officio is sparse, even though the rule itself derives from an old case of the English Court of Appeal (In re St. Nazaire Co. (1879), 12 Ch. D. 88). Essentially, the rule is that the court has no jurisdiction to reopen or amend a final decision, except in two cases: (1) where there has been a slip in drawing up the judgment, or (2) where there has been error in expressing the manifest intention of the court. More recently, this Court affirmed that this rule need not always be rigidly applied to tribunals in the administrative law context when the policy reasons for it are not present,
The existence and scope of a right of appeal has often been made the focus of analytical attention in applying the functus doctrine. Such was the case when the power of the Court of Chancery to rehear cases was extinguished by the Judicature Acts in 1873 by fusing common law and equity jurisdictions into one court and providing for a single appeal to a newly created Court of Appeal (In re St. Nazaire, supra). Originally, this was also the focus of the functus analysis for administrative tribunals that had rights of appeal tightly constrained by statute (see Grillas v. Minister of Manpower and Immigration, 1971 CanLII 3 (S.C.C.),  S.C.R. 577). However, the underlying rationale for the doctrine is clearly more fundamental: that for the due and proper administration of justice, there must be finality to a proceeding to ensure procedural fairness and the integrity of the judicial system. The point is plainly made by Sopinka J.
As a general rule, once . . . a tribunal has reached a final decision in respect to the matter that is before it in accordance with its enabling statute, that decision cannot be revisited because the tribunal has changed its mind, made an error within jurisdiction or because there has been a change of circumstances. . . .
To this extent, the principle of functus officio applies. It is based, however, on the policy ground which favours finality of proceedings rather than the rule which was developed with respect to formal judgments of a court whose decision was subject to a full appeal.
If a court is permitted to continually revisit or reconsider final orders simply because it has changed its mind or wishes to continue exercising jurisdiction over a matter, there would never be finality to a proceeding, or, as G. Pépin and Y. Ouellette have perceptively termed it, the providing of [translation] “legal security” for the parties. This concern for finality is evident in the definition of functus officio:
[translation] Qualifies a court or tribunal, a public body or an official that is no longer seized of a matter because it or he or she has discharged the office. E.g. A judge who has pronounced a final judgment is functus officio.
The principle ensures that subject to an appeal, parties are secure in their reliance on the finality of superior court decisions.
This common law rule is further reflected in modern rules of civil procedure (see, e.g., Nova Scotia Civil Procedure Rules, Rule 15.07) and the interpretation of criminal appeal provisions (see R. v. H. (E.F.) 1997 CanLII 418 (ON C.A.), (1997), 115 C.C.C. (3d) 89 (Ont. C.A.), considering s. 675 of the Criminal Code). Whether in its common law or statutory form, the doctrine of functus officio provides that only in strictly limited circumstances can a court revisit an order or judgment (see Nova Scotia Civil Procedure Rules, Rule 15.08). If it were otherwise, there would be, to paraphrase Charron J.A. in H. (E.F.), supra, at p. 101, the recurring danger of the trial process becoming or appearing to become a “never closing revolving door” through which litigants could come and go as they pleased.
In addition to this concern with finality, the question of whether a court is clothed with the requisite authority to act raises concerns related to the separation of powers, a principle that transcends procedural and common law rules. In our view, if a court intervenes, as here, in matters of administration properly entrusted to the executive, it exceeds its proper sphere and thereby breaches the separation of powers. By crossing the boundary between judicial acts and administrative oversight, it acts illegitimately and without jurisdiction. Such a crossing of the boundary cannot be characterized as relief that is “appropriate and just in the circumstances” within the meaning of s. 24(1) of the Charter.
-  2 S.C.R. 848
- see In re Swire (1885), 30 Ch. D. 239 (C.A.); Paper Machinery Ltd. v. J. O. Ross Engineering Corp., 1934 CanLII 1 (S.C.C.),  S.C.R. 186)
- Chandler v. Alberta Association of Architects, 1989 CanLII 41 (S.C.C.),  2 S.C.R 848
- in Chandler, supra, at pp. 861-62
- Principes de contentieux administratif (2nd ed. 1982), at p. 221
- H. Reid, Dictionnaire de droit québécois et canadien (2001), at p. 253
- "Functis Officio". Wingate. 15 August 2009. Retrieved 30 December 2013.