The Ventura County Star reports:

It turned out the law prevented Gallegly from withdrawing, and his name will appear on the ballot despite his desire to retire.
While contemplating his decision, Gallegly received incorrect advice from Ventura County Clerk-Recorder Phillip Schmit, who told Gallegly’s legal adviser, former District Attorney Mike Bradbury, that Gallegly would in fact be able to withdraw.
“You make decisions and, hopefully, they’re informed decisions,” Gallegly said. “I take responsibility for making the decision based on my understanding of the law that came from a person in authority.”
Schmit acknowledged he gave incorrect information to Bradbury in a phone conversation about 11:30 Friday morning.
“That was the original statement that, yes, he could withdraw,” Schmit said. “It was not the proper answer.”
After double-checking the law, however, Schmit left a voice-mail message on Bradbury’s phone about an hour later, informing him of the error.
Gallegly said he did not receive the message until after he had issued a news release and met with Star Editor Joe Howry to announce his decision to retire.
Gallegly and Bradbury were also under the impression that if the incumbent withdrew his nomination papers, other candidates would have an additional five days to come forward. That is the case with state and local offices, but not for members of Congress.
One of the reasons Gallegly believed such an extension would be granted is that when he initially ran for Congress, in 1986, a candidate who missed the filing period, Thomas La Porte, was granted an extension by a Ventura County Superior Court judge.
In that case, the extension was granted because La Porte had been told by a former county clerk that he would have additional time to file.
The case, however, did not set a precedent because of the unusual circumstance that La Porte had been given faulty information.
That decision was based on the legal concept of estoppel. “It’s the concept in law that the government is prevented from enforcing the law because it’s their mistake,” said Lance Olson, an attorney for the state Democratic Party and one of California’s leading experts on election law.

It doesn’t look like Gallegly would have a good estoppel argument, because he had already filed his nomination papers before he was given any incorrect legal advice by an elections official.