When can a defendant appeal after a conviction based on a plea of guilty or no contest?

When a defendant appeals after a conviction based on a plea of guilty or no contest, our review is limited to postconviction matters. As a general rule, "appeals may not be brought by defendants who have pleaded guilty or nolo contendere. [Penal Code section 1237.5] provides the general rule that defendants who have pleaded guilty must obtain a certificate of probable cause before they may bring an appeal." (People v. Hunter (2002) 100 Cal.App.4th 37, 41.)