St. Thomas Aquinas says that it is a mortal sin not to submit to an excommunication in which the authority doing it has a sufficient cause and the one receiving it is innocent (Summa Theologiæ, Supplement, Q. 21, Art. 4).
Kinds of Mistakes
First, he distinguishes between different kinds of mistakes in excommunicating, such as excommunicating 1) out of anger, "An excommunication may be unjust for two reasons. First, on the part of its author, as when anyone excommunicates through hatred or anger, and then, nevertheless, the excommunication takes effect, though its author sins, because the one who is excommunicated suffers justly, even if the author act wrongly in excommunicating him," or 2) from not observing the proper procedures, or 3) not having sufficient cause, "Secondly, on the part of the excommunication, through there being no proper cause, or through the sentence being passed without the forms of law being observed."
Unjust Excommunications Can Still Have Force
"If the error does not annul the sentence, this takes effect, and the person excommunicated should humbly submit (which will be credited to him as a merit), and either seek absolution from the person who has excommunicated him, or appeal to a higher judge. If, however, he were to contemn the sentence, he would 'ipso facto' sin mortally.
"But sometimes it happens that there is sufficient cause on the part of the excommunicator, but not on the part of the excommunicated, as when a man is excommunicated for a crime which he has not committed, but which has been proved against him: in this case, if he submit humbly, the merit of his humility will compensate him for the harm of excommunication."
Applications to Consecrating Bishops Against the Express Will of the Pope
Consecrating bishops against the express will of the Pope is one of the causes of excommunication, "Both the Bishop who, without a pontifical mandate, consecrates a person a Bishop, and the one who receives the consecration from him, incur a latae sententiae excommunication reserved to the Apostolic See" (Canon 1387). Therefore, this excommunication would fall into the category of those in which BOTH the excommunicator is right and the excommunicated deserves it.
For those who break this Canon and insist they are justified in doing so, St. Thomas still says they would be in mortal sin for disobeying it.