The document -- published on the San Mateo County court Web site -- was filed in July, the Santa Cruz Sentinel reported. All pages were stamped with the warning that it was under seal by court order.
All details of settlements between Pacific Gas and Electric and the families of the eight people killed in the explosion and fire and other victims had been kept secret.
Judge Steven Dylina, who is in charge of the case, said the Web page dealing with the explosion has been closed. Investigators are trying to determine how the information got published.
"From the clerk's office to the IT department, something happened," Dylina said. "I don't know where the glitch is."
Some victims whose claims are still outstanding have complained of the secrecy, saying they have no guidelines to help them decide whether to take their cases to trial.
PGE has settled with 100 of 400 plaintiffs, including all wrongful death claims. A company spokeswoman said that as of June 30 the legal case had cost it about $145 million, including settlements, a figure that is expected to be about $300 million more by the end of August.
The 2010 blast caused by a leak from a natural gas pipeline gutted 38 houses.