Gross error (erreur grossière) is the legal standard for challenging a decision rendered by a third-party expert appointed under Article 1592 of the French Civil Code. Absent contractual exception, the expert's determination is binding on the parties and subject to appeal only in cases of gross error — an error so obvious and contrary to professional standards that it deprives the decision of any probative value. This very high threshold gives the expert's decision near-finality, strengthening legal certainty in transactions but severely limiting remedies against incorrect methodology or contestable assumptions. It must be distinguished from simple methodological disagreement, which does not constitute gross error.

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Example: a court-appointed expert under Article 1592 values a services company using only the asset-based method, entirely excluding any earnings-based approach. The aggrieved shareholder challenges the decision for gross error before the court. Case law is divided: complete exclusion of an income approach for a service-intensive business may constitute gross error, while a mere disagreement over the weight attributed to each method does not.

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Hectelion documents all methodological choices rigorously in Article 1592 mandates to prevent any gross error challenge and provide parties with legally secure, definitive valuations.