When Contra Costa County District Attorney Mark Peterson appears before a judge Wednesday, he’ll be in the Martinez courtroom not as a prosecutor but as a defendant in a rare proceeding that could end in his removal from office.

A finding last month by a civil grand jury that Peterson engaged in misconduct and should be ousted kicked off a proceeding that hasn’t been seen against a California county prosecutor in decades, legal experts said.

“It happens three times in a century,” said Stanford law Professor Robert Weisberg. “It’s very, very rare.”

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While proceedings like those against Peterson are exceedingly rare, Weisberg said, it’s much more common for prosecutors to be disqualified from individual cases because of issues like conflict of interest.