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 Idaho Code § 28-1-103 specifies that 'unless displaced by the particular provisions of [the UCC], the principles of law and equity, including . . . mistake . . . [shall] supplement its provisions.' See also WHITE & SUMMERS, § 2-11 at 103. The common law mistake doctrine is not inconsistent with I.C. § 28-12-202 and therefore may be applied to transactions that are governed by the statute. The mistake exception makes extrinsic evidence admissible to show that by reason of either mutual mistake or one party's unilateral mistake which is known to the other party, the written agreement does not express the parties' ...