You and a friend shake hands on a deal, but later you find out you were both thinking of completely different things. Or someone pressures you into signing a contract by threatening to ruin your business. In those situations, the law steps in and says: “That agreement isn’t really fair — we might set it aside.” This chapter explores three main ways a contract can be undone: mistake, duress, and undue influence, plus the rare defence of non est factum.