Whether Mac can rescind the contract selling the painting. Rescission is cancelling the contract as if it had never existed. This is to be contrasted with termination which stops the contract at the time it is terminated. The act of rescission means that the parties are restored to the status quo prior to contract and the contract is treated as never having existed. Mistake can be the basis for rescission as certain types of mistakes can lead to contracts being declared void or unenforceable (as if they never existed). Mistakes can lead to void or unenforceable contracts when there is (a) mutual mistake or (b) when it can be inferred from the known facts that the consent or agreement to the transaction was intended to be conditional on the truth of the mistaken belief, or (c) when the common error makes the thing contracted for essentially different from the thing that it was believed to be. Lindy and Mac agreed on a sale of the painting and frame. There was no indication that they were mutually mistaken, nor made the transaction conditional on the truth of a mistaken belief (it was not conditioned that the painting has to be of a particular artist) nor was there a common error which relates to the quality (which was an essential term or condition) of the thing being sold (there was none on this regard!). In fact, Mac was already aware of the other painting (at the back) when he sold the painting and the frame to Lindy. There was no mistake as to what was being sold to Lindy. Since there was no mistake which would lead to the contract of sale being declared void or unenforceable, there is no basis for a rescission.