[gbcdoj]

In other words, the pope can be wrong in his judgment--and in such a case the innocent party would rightfully consider any excommunication a nullity.

Got a traditional source for this?

The Jansenist heretics dared to teach such doctrines as that an excommunication pronounced by a lawful prelate could be ignored on a pretext of injustice ... Our predecessor of happy memory Clement XI in his constitution Unigenitus against the errors of Quesnell forbade and condemned statements of this kind. (Bl. Pius IX, Quartus Supra)

[ultima ratio]

A source: the Catholic Encyclopedia:

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An excommunication is said to be null when it is invalid because of some intrinsic or essential defect, e.g. when the person inflicting it has no jurisdiction, when the motive of the excommunication is manifestly incorrect and inconsistent, or when the excommunication is essentially defective in form. Excommunication is said to be unjust when, though valid, it is wrongfully applied to a person really innocent but believed to be guilty.