Cellphones and other electronic devices are not permitted inside the courtroom where Supreme Court justices hear cases.
Even lawyers arguing cases before the justices are forbidden from bringing in their cellphones.
Before entering the courtroom, visitors must leave their phones in lockers and pass through metal detectors.
During Tuesday morning's arguments in the case of Bristol-Myers Squibb Co. v. Superior Court of California, the ring of a cellphone could be heard.
Justice Stephen Breyer quickly maneuvered to turn off the device, to his colleagues' amusement.
The Associated Press reports:
"Breyer and seatmate Justice Clarence Thomas shared a quick laugh about the incident. But it didn't fluster lawyer Neal Katyal, who was in the middle of answering a question."
Some questions about the incident remain unanswered.
The Washington Post reports:
"Breyer's ringtone was not the usual sounding one, but reporters couldn't make out exactly what it was. And it is unclear whether a justice's black robe has pockets."