Legal Question in Appeals and Writs in California

That depends on which court denied your petition. Saying that it was "in California" narrows it down, but there are at least five courts you could be talking about -- the Superior Court, the California Court of Appeal, the California Supreme Court, the U.S. District Court or the U.S. Court of Appeals for the Ninth Circuit. That is generally the order in which you will want to approach the courts, followed potentially by a petition to the U.S. Supreme Court.

Bear in mind that what you say was "clearly" an error may not have been an error at all, or at least not a clear one. Few laypeople understand the nuances of habeas corpus law, and fewer still can evaluate their own cases impartially. You should consult with an experienced appellate attorney as soon as possible.

Feel free to contact me directly if you would like to discuss your case in more detail.