Once in a while, a trial judge makes a mistake that will do irreparable damage unless it is fixed immediately. That is one rationale behind the "collateral order doctrine," a narrow exception to the final judgment rule that allows federal appeals courts to review certain trial court decisions before final judgment. The doctrine recognizes that some protections ? sovereign immunity and double jeopardy, for example ? can be effectively preserved only if immediate appellate review is allowed. It is time the U.S. Supreme Court added decisions denying claims of attorney-client privilege to the list of immediately appealable collateral orders ? something it will have occasion to do this term.