How many times have we watched a movie, read a book, or listened to a colleague talk about an action that appeared to be a no-risk proposition, only to turn into a nightmare? At some point, the general lament is uttered: “It seemed like a good idea at the time, but . . .” Tax plans can be like that, too. A company identifies an acquisition target, proposes a merger with a supplier, or considers an internal restructure. Teams of lawyers, accountants, and operations personnel perform appropriate due diligence. The deal closes. At some point, blemishes, problems, errors float to the surface. The same lament is uttered: “It seemed like a good idea at the time, but . . .” While the laments are the same, the suffering for a tax planning mistake need not linger forever. If the parties to a transaction act quickly, the doctrine of rescission may apply, allowing the parties to treat the event as if it never occurred. Stanley C. Ruchelman and Neha Rastogi explain the early cases and discuss a published ruling and several private letter rulings in which the principal concern of the I.R.S. is that the transaction and its rescission occur in the same taxable year.