From U.S. v. Alcantara (2d Cir. 2005):

Before: WALKER, Chief Judge, CARDAMONE, WINTER, STRAUB, and LAY,[*] Circuit Judges.

[*] The Honorable Donald P. Lay, Senior Circuit Judge, United States Court of Appeals for the Eighth Circuit, sitting by designation.

Five judges? How did that happen? Federal courts of appeal generally sit in three-judge panels; sometimes only two judges are listed, for instance if one of the judges couldn’t participate; some opinions are one-judge decisions on certain motions; and of course there are en bancs that generally include all the judges in the circuit (except in the Ninth Circuit, where those generally have only 11 of the judges), but this isn’t an en banc.

Post your answer in the comments, without peeking at the opinion. Or, if you just want to learn the answer for yourself, see here and here. I imagine this must have happened before, but it’s the first time I’ve seen it.