Void judgments (CCP 473(d)):
The court may, on its own motion or the motion of either party, set aside any void judgment or order. A judgment or order may be void if the issuing court lacked subject matter jurisdiction over the action, personal jurisdiction over the defendant, if the judgment or order granted relief that the court had no power to grant, or if the judgment was procured by fraud on the court.
A common reason default judgments are considered void is if the judgment was obtained after improper or fraudulent service of process, for example, falsified declarations related to personal service of the summons and complaint. Lack of proper service results in lack of personal jurisdiction over the defendant.
There are many other ways in which a default judgment may be void. For example, a default judgment for an amount in excess of the demand in the complaint is void, and for good reasons: the defendant was not notified of the amount demanded by the plaintiff, even if there was proper service of process.