When a separate and new lawsuit is filed to challenge some aspect of an earlier and separate case, it is called a collateral attack on the earlier case. This is different than an appeal, which is a challenge to some aspect of a decision made in the same case.
Example: Sam obtains a divorce in Nevada without properly notifying his wife, Laurie. Laurie files a later lawsuit seeking to set aside the divorce and start the divorce proceedings over. Laurie's case is a collateral attack on the divorce.
The law wants judgments to be final whenever possible, and thus collateral attacks are discouraged. Many are filed, but usually only succeed when an obvious injustice or unconstitutional treatment occurred in the earlier case.