The U.S. Courts of Appeals must ordinarily convene en banc to overturn circuit law. However, roughly half of the circuit courts have adopted an alternative, less costly procedure, the informal en banc, where three-judge panels may overturn precedent with approval of the full circuit. This article leverages variation in adoption and implementation of this institution to analyze the implications of ex post oversight mechanisms for ex ante panel decision making. The evidence suggests that the informal en banc substantially reduces the impact of ideology on panel decision making, providing new evidence that lower court judges strategically alter their behavior in anticipation of potential override by circuit colleagues.