Short answer. A shared misunderstanding of the law can be a ground to annul. Article 1334 of the Civil Code provides that mutual error as to the legal effect of an agreement may vitiate consent when the real purpose of the parties is frustrated. The error must be shared, and it must defeat what both sides were trying to achieve.
What the law says
Mutual error as to the legal effect of an agreement when the real purpose of the parties is frustrated, may vitiate consent.
Civil Code, Article 1334 — Mutual Error of Law. Read the full provision →
An exception to a very old rule
The general principle is that ignorance of the law excuses no one, and a party cannot escape a contract merely because they misjudged its legal consequences. Article 1334 carves out a narrow exception. Where both parties shared a wrong understanding of what their agreement would legally accomplish, and that error defeats the very purpose they were pursuing, consent may be treated as defective. The reasoning is that neither party got the agreement they thought they were making, so there is no reason to hold either of them to it.
Three requirements, all of them strict
First, the error must concern the legal effect of the agreement, not the facts and not the wisdom of the deal. Second, it must be mutual: if only one side was mistaken while the other understood the position correctly, the article does not apply, and the mistaken party is generally left with the bargain they made.
Third, the real purpose of the parties must be frustrated. It is not enough that the legal result differs a little from what was expected; the mistake must defeat the object of the agreement, so that performing it would give neither side what the contract was for.
What annulment gets you, and what it does not
Note the wording: consent may be vitiated. That is permissive, and a court weighs the circumstances rather than annulling automatically. Annulment is also not automatic in another sense — a contract with vitiated consent is valid until a court sets it aside, the action must be brought within the period the law allows, and conduct confirming the contract after discovering the error can bar the remedy entirely.
If annulment succeeds, both sides restore what they received. Where the misunderstanding concerns something correctable rather than fatal — a wrongly described property, an instrument that fails to express the true agreement — reformation of the document is usually the cheaper and more sensible route, and is worth exploring first.
What this rule will not rescue
It is worth being blunt about the cases the article does not help. A party who did not read the document, who relied on a friend’s assurance about what it meant, or who simply misjudged the tax or inheritance consequences of a deal, is not covered. Nor does it apply where the parties understood the law correctly and one of them merely regrets the bargain.
The way to prove a mutual error is documentary: drafts, messages and instructions showing what both sides believed the arrangement would achieve. Without that, the claim reduces to one party’s recollection, which rarely carries it.