Error of law injures. A mistake of the law has an injurious effect; that is, the party committing it must suffer the consequences.

Explanation

(English-origin maxim.) A party who acts under a mistaken understanding of the law bears the consequences of that mistake — ignorance of law is no excuse, and equity will not relieve against its effects.

Where this applies

• A business owner signs a contract thinking he is exempt from sales tax, but the court holds him to the contract because his misunderstanding of the tax code does not excuse his obligation.
• A homeowner builds an addition without a permit, believing in good faith that permits are only needed for commercial buildings; the city orders it removed, and the court refuses to grant relief for the legal error.
• A driver exceeds the speed limit in a school zone, honestly believing the zone was only active on weekdays; the court convicts him because a mistake about when the law applies is still a mistake of law.