R2d § 151

Element: a mistake is a belief that is not in accord with the facts.

Foundational definition. Note the temporal anchor: the mistake must exist at the time the contract was made (not later regret about market changes).

Common misunderstanding: students confuse mistake with misprediction. A mistake is about EXISTING facts wrongly believed (the cow is barren when she is fertile). A misprediction is about future events (the cow will not become valuable). Misprediction is not a § 151 mistake and is ordinarily allocated by §§ 261, 265 (impracticability/frustration). Keep the categories separate or the analysis fails immediately.