The International Criminal Court (ICC) has apologised after the public gallery heard the names of protected witnesses.
On Friday the prosecutor mentioned the names of several witnesses, thinking the microphones were off.
The blunder took place during the trial of former Ivorian president Laurent Gbagbo for crimes against humanity, charges he denies.
The judge said he did not know whether it was "recklessness, superficiality or stupidity" that caused the mistake.
Presiding Judge Cuno Tarfusser added that he did not want to "speculate about something else".
The ICC has ordered a formal inquiry.
The BBC's Anna Holligan in The Hague reports that the incident was relayed to the public gallery and the recordings have since spread on social media, and even appeared on YouTube.
Our correspondent adds that protecting witnesses is one of the key promises of the ICC, and the court goes to great lengths to shield the identities of sensitive witnesses from the public by pixellating their faces and disguising their voices.
In some cases, witnesses are even moved to a new country and given a new identity.
This is the highest profile trial yet for the ICC, which has only convicted two people, both Congolese warlords, since its establishment in 2002.
Mr Gbagbo, 70, and ex-militia leader Charles Ble Goude, 44, deny murder, rape, attempted murder and persecution in the violence after Ivory Coast's election in 2010.
Mr Gbagbo sparked a crisis in Ivory Coast after he refused to step down following his loss to Alassane Ouattara in the presidential vote.
There were bloody clashes between rival forces over five months in 2010 and 2011.
Some 3,000 people were killed.
At the start of the trial the prosecution said it planned to bring forward 138 witnesses.
The trial is expected to last three to four years.
Gbagbo: From professor to president
- Born in 1945, Mr Gbagbo's first career was in academia as a history professor
- He was jailed for two years in 1971 for "subversive" teaching
- By the 1980s he was heavily involved in trade union activities
- After years in exile, he returned to Ivory Coast to attend the founding congress of the Ivorian Popular Front in 1988
- Mr Gbagbo was one of the first to challenge Ivory Coast's founding President Felix Houphouet-Boigny, after multi-party politics were permitted
- Became president with the Ivorian Popular Front in 2000